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Message-ID: <20130908090448.GD17294@redhat.com>
Date: Sun, 8 Sep 2013 12:04:48 +0300
From: Gleb Natapov <gleb@...hat.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org,
kvm@...r.kernel.org,
Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2] KVM: mmu: allow page tables to be in read-only slots
On Thu, Sep 05, 2013 at 02:21:53PM +0200, Paolo Bonzini wrote:
> Page tables in a read-only memory slot will currently cause a triple
> fault when running with shadow paging, because the page walker uses
> gfn_to_hva and it fails on such a slot.
>
> TianoCore uses such a page table. The idea is that, on real hardware,
> the firmware can already run in 64-bit flat mode when setting up the
> memory controller. Real hardware seems to be fine with that as long as
> the accessed/dirty bits are set. Thus, this patch saves whether the
> slot is readonly, and later checks it when updating the accessed and
> dirty bits.
>
> Note that this scenario is not supported by NPT at all, as explained by
> comments in the code.
>
> Cc: stable@...r.kernel.org
> Cc: kvm@...r.kernel.org
> Cc: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
> Cc: Gleb Natapov <gleb@...hat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
I would prefer to change gfn_to_hva_read() to gfn_to_hva_prot() in this
patch already, it will not make it any bigger, but as long as API
renaming patch follows it is up to you.
Reviewed-by: Gleb Natapov <gleb@...hat.com>
> ---
> arch/x86/kvm/paging_tmpl.h | 20 +++++++++++++++++++-
> include/linux/kvm_host.h | 1 +
> virt/kvm/kvm_main.c | 14 +++++++++-----
> 3 files changed, 29 insertions(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index 0433301..aa18aca 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -99,6 +99,7 @@ struct guest_walker {
> pt_element_t prefetch_ptes[PTE_PREFETCH_NUM];
> gpa_t pte_gpa[PT_MAX_FULL_LEVELS];
> pt_element_t __user *ptep_user[PT_MAX_FULL_LEVELS];
> + bool pte_writable[PT_MAX_FULL_LEVELS];
> unsigned pt_access;
> unsigned pte_access;
> gfn_t gfn;
> @@ -235,6 +236,22 @@ static int FNAME(update_accessed_dirty_bits)(struct kvm_vcpu *vcpu,
> if (pte == orig_pte)
> continue;
>
> + /*
> + * If the slot is read-only, simply do not process the accessed
> + * and dirty bits. This is the correct thing to do if the slot
> + * is ROM, and page tables in read-as-ROM/write-as-MMIO slots
> + * are only supported if the accessed and dirty bits are already
> + * set in the ROM (so that MMIO writes are never needed).
> + *
> + * Note that NPT does not allow this at all and faults, since
> + * it always wants nested page table entries for the guest
> + * page tables to be writable. And EPT works but will simply
> + * overwrite the read-only memory to set the accessed and dirty
> + * bits.
> + */
> + if (unlikely(!walker->pte_writable[level - 1]))
> + continue;
> +
> ret = FNAME(cmpxchg_gpte)(vcpu, mmu, ptep_user, index, orig_pte, pte);
> if (ret)
> return ret;
> @@ -309,7 +326,8 @@ retry_walk:
> goto error;
> real_gfn = gpa_to_gfn(real_gfn);
>
> - host_addr = gfn_to_hva(vcpu->kvm, real_gfn);
> + host_addr = gfn_to_hva_read(vcpu->kvm, real_gfn,
> + &walker->pte_writable[walker->level - 1]);
> if (unlikely(kvm_is_error_hva(host_addr)))
> goto error;
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index ca645a0..22f9cdf 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -533,6 +533,7 @@ int gfn_to_page_many_atomic(struct kvm *kvm, gfn_t gfn, struct page **pages,
>
> struct page *gfn_to_page(struct kvm *kvm, gfn_t gfn);
> unsigned long gfn_to_hva(struct kvm *kvm, gfn_t gfn);
> +unsigned long gfn_to_hva_read(struct kvm *kvm, gfn_t gfn, bool *writable);
> unsigned long gfn_to_hva_memslot(struct kvm_memory_slot *slot, gfn_t gfn);
> void kvm_release_page_clean(struct page *page);
> void kvm_release_page_dirty(struct page *page);
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index f7e4334..418d037 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1078,11 +1078,15 @@ unsigned long gfn_to_hva(struct kvm *kvm, gfn_t gfn)
> EXPORT_SYMBOL_GPL(gfn_to_hva);
>
> /*
> - * The hva returned by this function is only allowed to be read.
> - * It should pair with kvm_read_hva() or kvm_read_hva_atomic().
> + * If writable is set to false, the hva returned by this function is only
> + * allowed to be read.
> */
> -static unsigned long gfn_to_hva_read(struct kvm *kvm, gfn_t gfn)
> +unsigned long gfn_to_hva_read(struct kvm *kvm, gfn_t gfn, bool *writable)
> {
> + struct kvm_memory_slot *slot = gfn_to_memslot(kvm, gfn);
> + if (writable)
> + *writable = !memslot_is_readonly(slot);
> +
> return __gfn_to_hva_many(gfn_to_memslot(kvm, gfn), gfn, NULL, false);
> }
>
> @@ -1450,7 +1454,7 @@ int kvm_read_guest_page(struct kvm *kvm, gfn_t gfn, void *data, int offset,
> int r;
> unsigned long addr;
>
> - addr = gfn_to_hva_read(kvm, gfn);
> + addr = gfn_to_hva_read(kvm, gfn, NULL);
> if (kvm_is_error_hva(addr))
> return -EFAULT;
> r = kvm_read_hva(data, (void __user *)addr + offset, len);
> @@ -1488,7 +1492,7 @@ int kvm_read_guest_atomic(struct kvm *kvm, gpa_t gpa, void *data,
> gfn_t gfn = gpa >> PAGE_SHIFT;
> int offset = offset_in_page(gpa);
>
> - addr = gfn_to_hva_read(kvm, gfn);
> + addr = gfn_to_hva_read(kvm, gfn, NULL);
> if (kvm_is_error_hva(addr))
> return -EFAULT;
> pagefault_disable();
> --
> 1.8.3.1
--
Gleb.
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