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Message-ID: <CAAhV-H4bEyeV7WkfSNBJnicMhhnSwj3PEr9K4ZpXwto1=JyWUw@mail.gmail.com>
Date: Tue, 16 Sep 2025 22:21:32 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Bibo Mao <maobibo@...ngson.cn>
Cc: WANG Xuerui <kernel@...0n.name>, kvm@...r.kernel.org, loongarch@...ts.linux.dev,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] LoongArch: KVM: Fix VM migration failure with PTW enabled
On Mon, Sep 15, 2025 at 9:22 AM Bibo Mao <maobibo@...ngson.cn> wrote:
>
>
>
> On 2025/9/14 上午9:57, Huacai Chen wrote:
> > Hi, Bibo,
> >
> > On Wed, Sep 10, 2025 at 3:14 PM Bibo Mao <maobibo@...ngson.cn> wrote:
> >>
> >> With PTW disabled system, bit Dirty is HW bit for page writing, however
> >> with PTW enabled system, bit Write is HW bit for page writing. Previously
> >> bit Write is treated as SW bit to record page writable attribute for fast
> >> page fault handling in the secondary MMU, however with PTW enabled machine,
> >> this bit is used by HW already.
> >>
> >> Here define KVM_PAGE_SOFT_WRITE with SW bit _PAGE_MODIFIED, so that it can
> >> work on both PTW disabled and enabled machines. And with HW write bit, both
> >> bit Dirty and Write is set or clear.
> >>
> >> Signed-off-by: Bibo Mao <maobibo@...ngson.cn>
> >> ---
> >> arch/loongarch/include/asm/kvm_mmu.h | 20 ++++++++++++++++----
> >> arch/loongarch/kvm/mmu.c | 8 ++++----
> >> 2 files changed, 20 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/arch/loongarch/include/asm/kvm_mmu.h b/arch/loongarch/include/asm/kvm_mmu.h
> >> index 099bafc6f797..efcd593c42b1 100644
> >> --- a/arch/loongarch/include/asm/kvm_mmu.h
> >> +++ b/arch/loongarch/include/asm/kvm_mmu.h
> >> @@ -16,6 +16,13 @@
> >> */
> >> #define KVM_MMU_CACHE_MIN_PAGES (CONFIG_PGTABLE_LEVELS - 1)
> >>
> >> +/*
> >> + * _PAGE_MODIFIED is SW pte bit, it records page ever written on host
> >> + * kernel, on secondary MMU it records page writable in order to fast
> >> + * path handling
> >> + */
> >> +#define KVM_PAGE_SOFT_WRITE _PAGE_MODIFIED
> > KVM_PAGE_WRITEABLE is more suitable.
> both are ok for me.
> >
> >> +
> >> #define _KVM_FLUSH_PGTABLE 0x1
> >> #define _KVM_HAS_PGMASK 0x2
> >> #define kvm_pfn_pte(pfn, prot) (((pfn) << PFN_PTE_SHIFT) | pgprot_val(prot))
> >> @@ -52,11 +59,16 @@ static inline void kvm_set_pte(kvm_pte_t *ptep, kvm_pte_t val)
> >> WRITE_ONCE(*ptep, val);
> >> }
> >>
> >> -static inline int kvm_pte_write(kvm_pte_t pte) { return pte & _PAGE_WRITE; }
> >> -static inline int kvm_pte_dirty(kvm_pte_t pte) { return pte & _PAGE_DIRTY; }
> >> +static inline int kvm_pte_soft_write(kvm_pte_t pte) { return pte & KVM_PAGE_SOFT_WRITE; }
> > The same, kvm_pte_mkwriteable() is more suitable.
> kvm_pte_writable() here ? and kvm_pte_mkwriteable() for the bellowing
> sentense.
>
> If so, that is ok, both are ok for me.
Yes.
> >
> >> +static inline int kvm_pte_dirty(kvm_pte_t pte) { return pte & __WRITEABLE; }
> > _PAGE_DIRTY and _PAGE_WRITE are always set/cleared at the same time,
> > so the old version still works.
> Although it is workable, I still want to remove single bit _PAGE_DIRTY
> checking here.
I want to check a single bit because "kvm_pte_write() return
_PAGE_WRITE and kvm_pte_dirty() return _PAGE_DIRTY" looks more
natural.
You may argue that kvm_pte_mkdirty() set both _PAGE_WRITE and
_PAGE_DIRTY so kvm_pte_dirty() should also return both. But I think
kvm_pte_mkdirty() in this patch is just a "reasonable optimization".
Because strictly speaking, we need both kvm_pte_mkdirty() and
kvm_pte_mkwrite() and call the pair when needed.
Huacai
>
> Regards
> Bibo Mao
> >
> >> static inline int kvm_pte_young(kvm_pte_t pte) { return pte & _PAGE_ACCESSED; }
> >> static inline int kvm_pte_huge(kvm_pte_t pte) { return pte & _PAGE_HUGE; }
> >>
> >> +static inline kvm_pte_t kvm_pte_mksoft_write(kvm_pte_t pte)
> >> +{
> >> + return pte | KVM_PAGE_SOFT_WRITE;
> >> +}
> >> +
> >> static inline kvm_pte_t kvm_pte_mkyoung(kvm_pte_t pte)
> >> {
> >> return pte | _PAGE_ACCESSED;
> >> @@ -69,12 +81,12 @@ static inline kvm_pte_t kvm_pte_mkold(kvm_pte_t pte)
> >>
> >> static inline kvm_pte_t kvm_pte_mkdirty(kvm_pte_t pte)
> >> {
> >> - return pte | _PAGE_DIRTY;
> >> + return pte | __WRITEABLE;
> >> }
> >>
> >> static inline kvm_pte_t kvm_pte_mkclean(kvm_pte_t pte)
> >> {
> >> - return pte & ~_PAGE_DIRTY;
> >> + return pte & ~__WRITEABLE;
> >> }
> >>
> >> static inline kvm_pte_t kvm_pte_mkhuge(kvm_pte_t pte)
> >> diff --git a/arch/loongarch/kvm/mmu.c b/arch/loongarch/kvm/mmu.c
> >> index ed956c5cf2cc..68749069290f 100644
> >> --- a/arch/loongarch/kvm/mmu.c
> >> +++ b/arch/loongarch/kvm/mmu.c
> >> @@ -569,7 +569,7 @@ static int kvm_map_page_fast(struct kvm_vcpu *vcpu, unsigned long gpa, bool writ
> >> /* Track access to pages marked old */
> >> new = kvm_pte_mkyoung(*ptep);
> >> if (write && !kvm_pte_dirty(new)) {
> >> - if (!kvm_pte_write(new)) {
> >> + if (!kvm_pte_soft_write(new)) {
> >> ret = -EFAULT;
> >> goto out;
> >> }
> >> @@ -856,9 +856,9 @@ static int kvm_map_page(struct kvm_vcpu *vcpu, unsigned long gpa, bool write)
> >> prot_bits |= _CACHE_SUC;
> >>
> >> if (writeable) {
> >> - prot_bits |= _PAGE_WRITE;
> >> + prot_bits = kvm_pte_mksoft_write(prot_bits);
> >> if (write)
> >> - prot_bits |= __WRITEABLE;
> >> + prot_bits = kvm_pte_mkdirty(prot_bits);
> >> }
> >>
> >> /* Disable dirty logging on HugePages */
> >> @@ -904,7 +904,7 @@ static int kvm_map_page(struct kvm_vcpu *vcpu, unsigned long gpa, bool write)
> >> kvm_release_faultin_page(kvm, page, false, writeable);
> >> spin_unlock(&kvm->mmu_lock);
> >>
> >> - if (prot_bits & _PAGE_DIRTY)
> >> + if (kvm_pte_dirty(prot_bits))
> >> mark_page_dirty_in_slot(kvm, memslot, gfn);
> >>
> >> out:
> > To save time, I just change the whole patch like this, you can confirm
> > whether it woks:
> >
> > diff --git a/arch/loongarch/include/asm/kvm_mmu.h
> > b/arch/loongarch/include/asm/kvm_mmu.h
> > index 099bafc6f797..882f60c72b46 100644
> > --- a/arch/loongarch/include/asm/kvm_mmu.h
> > +++ b/arch/loongarch/include/asm/kvm_mmu.h
> > @@ -16,6 +16,13 @@
> > */
> > #define KVM_MMU_CACHE_MIN_PAGES (CONFIG_PGTABLE_LEVELS - 1)
> >
> > +/*
> > + * _PAGE_MODIFIED is SW pte bit, it records page ever written on host
> > + * kernel, on secondary MMU it records page writable in order to fast
> > + * path handling
> > + */
> > +#define KVM_PAGE_WRITEABLE _PAGE_MODIFIED
> > +
> > #define _KVM_FLUSH_PGTABLE 0x1
> > #define _KVM_HAS_PGMASK 0x2
> > #define kvm_pfn_pte(pfn, prot) (((pfn) << PFN_PTE_SHIFT) |
> > pgprot_val(prot))
> > @@ -56,6 +63,7 @@ static inline int kvm_pte_write(kvm_pte_t pte) {
> > return pte & _PAGE_WRITE; }
> > static inline int kvm_pte_dirty(kvm_pte_t pte) { return pte &
> > _PAGE_DIRTY; }
> > static inline int kvm_pte_young(kvm_pte_t pte) { return pte &
> > _PAGE_ACCESSED; }
> > static inline int kvm_pte_huge(kvm_pte_t pte) { return pte &
> > _PAGE_HUGE; }
> > +static inline int kvm_pte_writeable(kvm_pte_t pte) { return pte &
> > KVM_PAGE_WRITEABLE; }
> >
> > static inline kvm_pte_t kvm_pte_mkyoung(kvm_pte_t pte)
> > {
> > @@ -69,12 +77,12 @@ static inline kvm_pte_t kvm_pte_mkold(kvm_pte_t
> > pte)
> >
> > static inline kvm_pte_t kvm_pte_mkdirty(kvm_pte_t pte)
> > {
> > - return pte | _PAGE_DIRTY;
> > + return pte | __WRITEABLE;
> > }
> >
> > static inline kvm_pte_t kvm_pte_mkclean(kvm_pte_t pte)
> > {
> > - return pte & ~_PAGE_DIRTY;
> > + return pte & ~__WRITEABLE;
> > }
> >
> > static inline kvm_pte_t kvm_pte_mkhuge(kvm_pte_t pte)
> > @@ -87,6 +95,11 @@ static inline kvm_pte_t kvm_pte_mksmall(kvm_pte_t
> > pte)
> > return pte & ~_PAGE_HUGE;
> > }
> >
> > +static inline kvm_pte_t kvm_pte_mkwriteable(kvm_pte_t pte)
> > +{
> > + return pte | KVM_PAGE_WRITEABLE;
> > +}
> > +
> > static inline int kvm_need_flush(kvm_ptw_ctx *ctx)
> > {
> > return ctx->flag & _KVM_FLUSH_PGTABLE;
> > diff --git a/arch/loongarch/kvm/mmu.c b/arch/loongarch/kvm/mmu.c
> > index ed956c5cf2cc..7c8143e79c12 100644
> > --- a/arch/loongarch/kvm/mmu.c
> > +++ b/arch/loongarch/kvm/mmu.c
> > @@ -569,7 +569,7 @@ static int kvm_map_page_fast(struct kvm_vcpu
> > *vcpu, unsigned long gpa, bool writ
> > /* Track access to pages marked old */
> > new = kvm_pte_mkyoung(*ptep);
> > if (write && !kvm_pte_dirty(new)) {
> > - if (!kvm_pte_write(new)) {
> > + if (!kvm_pte_writeable(new)) {
> > ret = -EFAULT;
> > goto out;
> > }
> > @@ -856,9 +856,9 @@ static int kvm_map_page(struct kvm_vcpu *vcpu,
> > unsigned long gpa, bool write)
> > prot_bits |= _CACHE_SUC;
> >
> > if (writeable) {
> > - prot_bits |= _PAGE_WRITE;
> > + prot_bits = kvm_pte_mkwriteable(prot_bits);
> > if (write)
> > - prot_bits |= __WRITEABLE;
> > + prot_bits = kvm_pte_mkdirty(prot_bits);
> > }
> >
> > /* Disable dirty logging on HugePages */
> > @@ -904,7 +904,7 @@ static int kvm_map_page(struct kvm_vcpu *vcpu,
> > unsigned long gpa, bool write)
> > kvm_release_faultin_page(kvm, page, false, writeable);
> > spin_unlock(&kvm->mmu_lock);
> >
> > - if (prot_bits & _PAGE_DIRTY)
> > + if (kvm_pte_dirty(prot_bits))
> > mark_page_dirty_in_slot(kvm, memslot, gfn);
> >
> > out:
> >
> > Huacai
> >
> >>
> >> base-commit: 9dd1835ecda5b96ac88c166f4a87386f3e727bd9
> >> --
> >> 2.39.3
> >>
> >>
>
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