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Message-ID: <YPWcBGdgaLRFtJf8@google.com>
Date: Mon, 19 Jul 2021 16:36:36 +0100
From: Quentin Perret <qperret@...gle.com>
To: Marc Zyngier <maz@...nel.org>
Cc: james.morse@....com, alexandru.elisei@....com,
suzuki.poulose@....com, catalin.marinas@....com, will@...nel.org,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
linux-kernel@...r.kernel.org, ardb@...nel.org, qwandor@...gle.com,
tabba@...gle.com, dbrazdil@...gle.com, kernel-team@...roid.com
Subject: Re: [PATCH 07/14] KVM: arm64: Enable forcing page-level stage-2
mappings
On Monday 19 Jul 2021 at 15:24:32 (+0100), Marc Zyngier wrote:
> On Mon, 19 Jul 2021 11:47:28 +0100,
> Quentin Perret <qperret@...gle.com> wrote:
> >
> > Much of the stage-2 manipulation logic relies on being able to destroy
> > block mappings if e.g. installing a smaller mapping in the range. The
> > rationale for this behaviour is that stage-2 mappings can always be
> > re-created lazily. However, this gets more complicated when the stage-2
> > page-table is used to store metadata about the underlying pages. In such
> > a case, destroying a block mapping may lead to losing part of the
> > state, and confuse the user of those metadata (such as the hypervisor in
> > nVHE protected mode).
> >
> > To fix this, introduce a callback function in the pgtable struct which
> > is called during all map operations to determine whether the mappings
> > can us blocks, or should be forced to page-granularity level. This is
>
> nit: use?
Ack.
> > used by the hypervisor when creating the host stage-2 to force
> > page-level mappings when using non-default protection attributes.
> >
> > Signed-off-by: Quentin Perret <qperret@...gle.com>
> > ---
> > arch/arm64/include/asm/kvm_pgtable.h | 63 +++++++++++++++++----------
> > arch/arm64/kvm/hyp/nvhe/mem_protect.c | 16 +++++--
> > arch/arm64/kvm/hyp/pgtable.c | 20 +++++++--
> > 3 files changed, 69 insertions(+), 30 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> > index af62203d2f7a..dd72653314c7 100644
> > --- a/arch/arm64/include/asm/kvm_pgtable.h
> > +++ b/arch/arm64/include/asm/kvm_pgtable.h
> > @@ -75,25 +75,6 @@ enum kvm_pgtable_stage2_flags {
> > KVM_PGTABLE_S2_IDMAP = BIT(1),
> > };
> >
> > -/**
> > - * struct kvm_pgtable - KVM page-table.
> > - * @ia_bits: Maximum input address size, in bits.
> > - * @start_level: Level at which the page-table walk starts.
> > - * @pgd: Pointer to the first top-level entry of the page-table.
> > - * @mm_ops: Memory management callbacks.
> > - * @mmu: Stage-2 KVM MMU struct. Unused for stage-1 page-tables.
> > - */
> > -struct kvm_pgtable {
> > - u32 ia_bits;
> > - u32 start_level;
> > - kvm_pte_t *pgd;
> > - struct kvm_pgtable_mm_ops *mm_ops;
> > -
> > - /* Stage-2 only */
> > - struct kvm_s2_mmu *mmu;
> > - enum kvm_pgtable_stage2_flags flags;
> > -};
> > -
> > /**
> > * enum kvm_pgtable_prot - Page-table permissions and attributes.
> > * @KVM_PGTABLE_PROT_X: Execute permission.
> > @@ -109,11 +90,41 @@ enum kvm_pgtable_prot {
> > KVM_PGTABLE_PROT_DEVICE = BIT(3),
> > };
> >
> > -#define PAGE_HYP (KVM_PGTABLE_PROT_R | KVM_PGTABLE_PROT_W)
> > +#define KVM_PGTABLE_PROT_RW (KVM_PGTABLE_PROT_R | KVM_PGTABLE_PROT_W)
> > +#define KVM_PGTABLE_PROT_RWX (KVM_PGTABLE_PROT_RW | KVM_PGTABLE_PROT_X)
> > +
> > +#define PAGE_HYP KVM_PGTABLE_PROT_RW
> > #define PAGE_HYP_EXEC (KVM_PGTABLE_PROT_R | KVM_PGTABLE_PROT_X)
> > #define PAGE_HYP_RO (KVM_PGTABLE_PROT_R)
> > #define PAGE_HYP_DEVICE (PAGE_HYP | KVM_PGTABLE_PROT_DEVICE)
> >
> > +typedef bool (*kvm_pgtable_want_pte_cb_t)(u64 addr, u64 end,
> > + enum kvm_pgtable_prot prot);
> > +
> > +/**
> > + * struct kvm_pgtable - KVM page-table.
> > + * @ia_bits: Maximum input address size, in bits.
> > + * @start_level: Level at which the page-table walk starts.
> > + * @pgd: Pointer to the first top-level entry of the page-table.
> > + * @mm_ops: Memory management callbacks.
> > + * @mmu: Stage-2 KVM MMU struct. Unused for stage-1 page-tables.
> > + * @flags: Stage-2 page-table flags.
> > + * @want_pte_cb: Callback function used during map operations to decide
> > + * whether block mappings can be used to map the given IPA
> > + * range.
> > + */
> > +struct kvm_pgtable {
> > + u32 ia_bits;
> > + u32 start_level;
> > + kvm_pte_t *pgd;
> > + struct kvm_pgtable_mm_ops *mm_ops;
> > +
> > + /* Stage-2 only */
> > + struct kvm_s2_mmu *mmu;
> > + enum kvm_pgtable_stage2_flags flags;
> > + kvm_pgtable_want_pte_cb_t want_pte_cb;
> > +};
>
> nit: does this whole definition really need to move around?
The alternative is to move (or forward declare) enum kvm_pgtable_prot
higher up in the file, but I have no strong opinion, so whatever you
prefer will work for me.
> > +
> > /**
> > * struct kvm_mem_range - Range of Intermediate Physical Addresses
> > * @start: Start of the range.
> > @@ -216,21 +227,25 @@ int kvm_pgtable_hyp_map(struct kvm_pgtable *pgt, u64 addr, u64 size, u64 phys,
> > u64 kvm_get_vtcr(u64 mmfr0, u64 mmfr1, u32 phys_shift);
> >
> > /**
> > - * kvm_pgtable_stage2_init_flags() - Initialise a guest stage-2 page-table.
> > + * kvm_pgtable_stage2_init_full() - Initialise a guest stage-2 page-table.
> > * @pgt: Uninitialised page-table structure to initialise.
> > * @arch: Arch-specific KVM structure representing the guest virtual
> > * machine.
> > * @mm_ops: Memory management callbacks.
> > * @flags: Stage-2 configuration flags.
> > + * @want_pte_cb: Callback function used during map operations to decide
> > + * whether block mappings can be used to map the given IPA
> > + * range.
> > *
> > * Return: 0 on success, negative error code on failure.
> > */
> > -int kvm_pgtable_stage2_init_flags(struct kvm_pgtable *pgt, struct kvm_arch *arch,
> > +int kvm_pgtable_stage2_init_full(struct kvm_pgtable *pgt, struct kvm_arch *arch,
> > struct kvm_pgtable_mm_ops *mm_ops,
> > - enum kvm_pgtable_stage2_flags flags);
> > + enum kvm_pgtable_stage2_flags flags,
> > + kvm_pgtable_want_pte_cb_t want_pte_cb);
> >
> > #define kvm_pgtable_stage2_init(pgt, arch, mm_ops) \
> > - kvm_pgtable_stage2_init_flags(pgt, arch, mm_ops, 0)
> > + kvm_pgtable_stage2_init_full(pgt, arch, mm_ops, 0, NULL)
>
> nit: in general, we use __foo() as the primitive for foo(), rather
> than foo_with_icing_on_top().
Sure.
> >
> > /**
> > * kvm_pgtable_stage2_destroy() - Destroy an unused guest stage-2 page-table.
> > diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > index 58edc62be6f7..cdace80d3e28 100644
> > --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> > @@ -89,6 +89,7 @@ static void prepare_host_vtcr(void)
> > id_aa64mmfr1_el1_sys_val, phys_shift);
> > }
> >
> > +static bool host_stage2_want_pte_cb(u64 addr, u64 end, enum kvm_pgtable_prot prot);
> > int kvm_host_prepare_stage2(void *pgt_pool_base)
> > {
> > struct kvm_s2_mmu *mmu = &host_kvm.arch.mmu;
> > @@ -101,8 +102,9 @@ int kvm_host_prepare_stage2(void *pgt_pool_base)
> > if (ret)
> > return ret;
> >
> > - ret = kvm_pgtable_stage2_init_flags(&host_kvm.pgt, &host_kvm.arch,
> > - &host_kvm.mm_ops, KVM_HOST_S2_FLAGS);
> > + ret = kvm_pgtable_stage2_init_full(&host_kvm.pgt, &host_kvm.arch,
> > + &host_kvm.mm_ops, KVM_HOST_S2_FLAGS,
> > + host_stage2_want_pte_cb);
> > if (ret)
> > return ret;
> >
> > @@ -225,9 +227,17 @@ static inline int __host_stage2_idmap(u64 start, u64 end,
> > __ret; \
> > })
> >
> > +static bool host_stage2_want_pte_cb(u64 addr, u64 end, enum kvm_pgtable_prot prot)
> > +{
> > + if (range_is_memory(addr, end))
> > + return prot != KVM_PGTABLE_PROT_RWX;
> > + else
> > + return prot != KVM_PGTABLE_PROT_RW;
> > +}
>
> This really deserves a comment about *why* you make such decision.
> also find it a bit odd that you use the permissions to decide whether
> to map a block or a not. It feels like the permission is more of a
> side effect than anything else.
The idea is to use page-level mappings for anything that we can't
rebuild lazily in the host stage-2. So the logic in this function
matches exactly what we do in host_stage2_idmap() just below.
And the protection does matter sadly. If for instance we map a large
portion of the host RO, and we use a block mapping for that, then any
subsequent map() call in the block range is likely to have very
undesirable side effects -- we'll forget that the rest of the block
range should be RO and we risk mapping it RW(X) in the mem abort path.
But yes, a comment is very much needed, I'll add something.
> > static int host_stage2_idmap(u64 addr)
> > {
> > - enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R | KVM_PGTABLE_PROT_W;
> > + enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_RW;
> > struct kvm_mem_range range;
> > bool is_memory = find_mem_range(addr, &range);
> > int ret;
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 34cf67997a82..5bdbe7a31551 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -452,6 +452,8 @@ int kvm_pgtable_hyp_init(struct kvm_pgtable *pgt, u32 va_bits,
> > pgt->start_level = KVM_PGTABLE_MAX_LEVELS - levels;
> > pgt->mm_ops = mm_ops;
> > pgt->mmu = NULL;
> > + pgt->want_pte_cb = NULL;
> > +
> > return 0;
> > }
> >
> > @@ -491,6 +493,7 @@ struct stage2_map_data {
> > struct kvm_pgtable_mm_ops *mm_ops;
> >
> > int ret;
> > + bool force_pte;
>
> OK, so you have *two* mechanisms here: once to decide if a range can
> be mapped as a block or not, and another one to remember the result
> while walking the S2 PTW. This probably deserves some documentation
> and/or patch splitting.
Sure, I'll add a comment.
> > };
> >
> > u64 kvm_get_vtcr(u64 mmfr0, u64 mmfr1, u32 phys_shift)
> > @@ -613,6 +616,9 @@ static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> > struct kvm_pgtable *pgt = data->mmu->pgt;
> > struct kvm_pgtable_mm_ops *mm_ops = data->mm_ops;
> >
> > + if (data->force_pte && (level < (KVM_PGTABLE_MAX_LEVELS - 1)))
> > + return -E2BIG;
> > +
> > if (!kvm_block_mapping_supported(addr, end, phys, level))
> > return -E2BIG;
> >
> > @@ -660,6 +666,9 @@ static int stage2_map_walk_table_pre(u64 addr, u64 end, u32 level,
> > if (data->anchor)
> > return 0;
> >
> > + if (data->force_pte && (level < (KVM_PGTABLE_MAX_LEVELS - 1)))
> > + return 0;
> > +
> > if (!kvm_block_mapping_supported(addr, end, data->phys, level))
>
> There is something in me screaming that kvm_block_mapping_supported()
> should be the point where we check for these things... Or at least a
> helper function that takes 'data' as a parameter.
I feel like kvm_block_mapping_supported() might be better as-is as a
purely architectural check that we can also use it for e.g. hyp stage-1
stuff, but a new helper function should do.
>
> > return 0;
> >
> > @@ -791,6 +800,7 @@ int kvm_pgtable_stage2_map(struct kvm_pgtable *pgt, u64 addr, u64 size,
> > .memcache = mc,
> > .mm_ops = pgt->mm_ops,
> > .ret = 0,
> > + .force_pte = pgt->want_pte_cb && pgt->want_pte_cb(addr, addr + size, prot),
>
> Reading this makes me want to rename want_pte_cb() to force_pte_cb()...
No objection from me, I'll rename.
> > };
> > struct kvm_pgtable_walker walker = {
> > .cb = stage2_map_walker,
> > @@ -826,6 +836,7 @@ int kvm_pgtable_stage2_set_owner(struct kvm_pgtable *pgt, u64 addr, u64 size,
> > .mm_ops = pgt->mm_ops,
> > .owner_id = owner_id,
> > .ret = 0,
> > + .force_pte = true,
> > };
> > struct kvm_pgtable_walker walker = {
> > .cb = stage2_map_walker,
> > @@ -1070,9 +1081,11 @@ int kvm_pgtable_stage2_flush(struct kvm_pgtable *pgt, u64 addr, u64 size)
> > return kvm_pgtable_walk(pgt, addr, size, &walker);
> > }
> >
> > -int kvm_pgtable_stage2_init_flags(struct kvm_pgtable *pgt, struct kvm_arch *arch,
> > - struct kvm_pgtable_mm_ops *mm_ops,
> > - enum kvm_pgtable_stage2_flags flags)
> > +
> > +int kvm_pgtable_stage2_init_full(struct kvm_pgtable *pgt, struct kvm_arch *arch,
> > + struct kvm_pgtable_mm_ops *mm_ops,
> > + enum kvm_pgtable_stage2_flags flags,
> > + kvm_pgtable_want_pte_cb_t want_pte_cb)
> > {
> > size_t pgd_sz;
> > u64 vtcr = arch->vtcr;
> > @@ -1090,6 +1103,7 @@ int kvm_pgtable_stage2_init_flags(struct kvm_pgtable *pgt, struct kvm_arch *arch
> > pgt->mm_ops = mm_ops;
> > pgt->mmu = &arch->mmu;
> > pgt->flags = flags;
> > + pgt->want_pte_cb = want_pte_cb;
> >
> > /* Ensure zeroed PGD pages are visible to the hardware walker */
> > dsb(ishst);
Cheers,
Quentin
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