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Message-ID: <CAAhV-H65H_iREuETGU_v9oZdaPFoQj1VZV46XSNTC8ppENXzuQ@mail.gmail.com>
Date: Sun, 14 Sep 2025 09:57:08 +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
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.
> +
> #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.
> +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.
> 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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