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Message-ID: <3d3a72c2-7c91-7640-5f0b-7b95bd5f0d2e@loongson.cn>
Date: Mon, 15 Sep 2025 09:20:37 +0800
From: Bibo Mao <maobibo@...ngson.cn>
To: Huacai Chen <chenhuacai@...nel.org>
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 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.
>
>> +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.
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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