* Re: [PATCH] LoongArch: KVM: Preserve memslot arch flags on KVM_MR_FLAGS_ONLY
2026-08-28 9:48 [PATCH] LoongArch: KVM: Preserve memslot arch flags on KVM_MR_FLAGS_ONLY Zeng Chi
@ 2026-08-31 2:09 ` Bibo Mao
2026-08-31 6:30 ` Tao Cui
2026-09-05 15:08 ` Huacai Chen
2 siblings, 0 replies; 4+ messages in thread
From: Bibo Mao @ 2026-08-31 2:09 UTC (permalink / raw)
To: Zeng Chi, zhaotianrui, chenhuacai
Cc: kvm, loongarch, linux-kernel, zengchi, stable
On 2026/8/28 下午5:48, Zeng Chi wrote:
> From: Zeng Chi <zengchi@kylinos.cn>
>
> kvm_arch_prepare_memory_region() computes new->arch.flags, i.e. whether
> a memslot is KVM_MEM_HUGEPAGE_CAPABLE or KVM_MEM_HUGEPAGE_INCAPABLE, only
> for KVM_MR_CREATE and KVM_MR_MOVE, and returns early for every other
> change. But the generic code allocates a zeroed memslot for every
> change and never copies old->arch, so after a KVM_MR_FLAGS_ONLY update,
> e.g. toggling KVM_MEM_LOG_DIRTY_PAGES for live migration, the active
> memslot has arch.flags == 0.
>
> With both flags clear, fault_supports_huge_mapping() falls through to
> the alignment check on the HVA range alone, which no longer verifies
> that the GPA and HVA have the same offset within a PMD. A memslot that
> was marked KVM_MEM_HUGEPAGE_INCAPABLE because of a GPA/HVA offset
> mismatch can then be mapped with PMD entries on read faults, and since
> kvm_map_page() aligns the gfn and the pfn independently, the guest ends
> up accessing the wrong host pages, exactly the "d -> f, e -> g" case
> described in the comment above the check.
>
> Carry the arch flags over from the old memslot for KVM_MR_FLAGS_ONLY,
> as the GPA, HVA and size are guaranteed to be unchanged for that case.
>
> Fixes: 7ab6fb505b2a ("LoongArch: KVM: Optimization for memslot hugepage checking")
> Cc: stable@vger.kernel.org
> Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
> ---
> arch/loongarch/kvm/mmu.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/arch/loongarch/kvm/mmu.c b/arch/loongarch/kvm/mmu.c
> index e104897aa532..4ca7c6ea8f08 100644
> --- a/arch/loongarch/kvm/mmu.c
> +++ b/arch/loongarch/kvm/mmu.c
> @@ -383,6 +383,16 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm, const struct kvm_memory_slot
> hva_t hva_start;
> size_t size, gpa_offset, hva_offset;
>
> + /*
> + * The generic code allocates a fresh, zeroed memslot for every change,
> + * so the arch flags computed below must be carried over when only the
> + * userspace flags change, e.g. when dirty logging is toggled.
> + */
> + if (change == KVM_MR_FLAGS_ONLY) {
> + new->arch = old->arch;
> + return 0;
> + }
> +
> if ((change != KVM_MR_MOVE) && (change != KVM_MR_CREATE))
> return 0;
> /*
>
>
yes, when VM starts to migrate and set KVM_MEM_LOG_DIRTY_PAGES flag, or
cancel migration and clear KVM_MEM_LOG_DIRTY_PAGES flag. The arch
specified flag in new memslot should be kept the same. Thanks for the
contribution.
Reviewed-by: Bibo Mao <maobibo@loongson.cn>
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [PATCH] LoongArch: KVM: Preserve memslot arch flags on KVM_MR_FLAGS_ONLY
2026-08-28 9:48 [PATCH] LoongArch: KVM: Preserve memslot arch flags on KVM_MR_FLAGS_ONLY Zeng Chi
2026-08-31 2:09 ` Bibo Mao
@ 2026-08-31 6:30 ` Tao Cui
2026-09-05 15:08 ` Huacai Chen
2 siblings, 0 replies; 4+ messages in thread
From: Tao Cui @ 2026-08-31 6:30 UTC (permalink / raw)
To: Zeng Chi, zhaotianrui, maobibo, chenhuacai
Cc: cui.tao, kvm, loongarch, linux-kernel, zengchi, stable
在 2026/8/28 17:48, Zeng Chi 写道:
> From: Zeng Chi <zengchi@kylinos.cn>
>
> kvm_arch_prepare_memory_region() computes new->arch.flags, i.e. whether
> a memslot is KVM_MEM_HUGEPAGE_CAPABLE or KVM_MEM_HUGEPAGE_INCAPABLE, only
> for KVM_MR_CREATE and KVM_MR_MOVE, and returns early for every other
> change. But the generic code allocates a zeroed memslot for every
> change and never copies old->arch, so after a KVM_MR_FLAGS_ONLY update,
> e.g. toggling KVM_MEM_LOG_DIRTY_PAGES for live migration, the active
> memslot has arch.flags == 0.
>
> With both flags clear, fault_supports_huge_mapping() falls through to
> the alignment check on the HVA range alone, which no longer verifies
> that the GPA and HVA have the same offset within a PMD. A memslot that
> was marked KVM_MEM_HUGEPAGE_INCAPABLE because of a GPA/HVA offset
> mismatch can then be mapped with PMD entries on read faults, and since
> kvm_map_page() aligns the gfn and the pfn independently, the guest ends
> up accessing the wrong host pages, exactly the "d -> f, e -> g" case
> described in the comment above the check.
>
> Carry the arch flags over from the old memslot for KVM_MR_FLAGS_ONLY,
> as the GPA, HVA and size are guaranteed to be unchanged for that case.
>
> Fixes: 7ab6fb505b2a ("LoongArch: KVM: Optimization for memslot hugepage checking")
> Cc: stable@vger.kernel.org
> Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
> ---
> arch/loongarch/kvm/mmu.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/arch/loongarch/kvm/mmu.c b/arch/loongarch/kvm/mmu.c
> index e104897aa532..4ca7c6ea8f08 100644
> --- a/arch/loongarch/kvm/mmu.c
> +++ b/arch/loongarch/kvm/mmu.c
> @@ -383,6 +383,16 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm, const struct kvm_memory_slot
> hva_t hva_start;
> size_t size, gpa_offset, hva_offset;
>
> + /*
> + * The generic code allocates a fresh, zeroed memslot for every change,
> + * so the arch flags computed below must be carried over when only the
> + * userspace flags change, e.g. when dirty logging is toggled.
> + */
> + if (change == KVM_MR_FLAGS_ONLY) {
> + new->arch = old->arch;
> + return 0;
> + }
> +
> if ((change != KVM_MR_MOVE) && (change != KVM_MR_CREATE))
> return 0;
> /*
I verified this on a Loongson-3A6000 host with a KVM selftest that uses
a GPA/HVA offset-mismatched memslot and toggles dirty logging: the
unpatched kernel maps the wrong host pages into the guest after the
toggle, and with this patch the mapping stays correct.
Tested-by: Tao Cui <cuitao@kylinos.cn>
Reviewed-by: Tao Cui <cuitao@kylinos.cn>
^ permalink raw reply [flat|nested] 4+ messages in thread* Re: [PATCH] LoongArch: KVM: Preserve memslot arch flags on KVM_MR_FLAGS_ONLY
2026-08-28 9:48 [PATCH] LoongArch: KVM: Preserve memslot arch flags on KVM_MR_FLAGS_ONLY Zeng Chi
2026-08-31 2:09 ` Bibo Mao
2026-08-31 6:30 ` Tao Cui
@ 2026-09-05 15:08 ` Huacai Chen
2 siblings, 0 replies; 4+ messages in thread
From: Huacai Chen @ 2026-09-05 15:08 UTC (permalink / raw)
To: Zeng Chi
Cc: zhaotianrui, maobibo, kvm, loongarch, linux-kernel, zengchi,
stable
Applied, thanks.
Huacai
On Fri, Aug 28, 2026 at 5:48 PM Zeng Chi <zeng_chi911@163.com> wrote:
>
> From: Zeng Chi <zengchi@kylinos.cn>
>
> kvm_arch_prepare_memory_region() computes new->arch.flags, i.e. whether
> a memslot is KVM_MEM_HUGEPAGE_CAPABLE or KVM_MEM_HUGEPAGE_INCAPABLE, only
> for KVM_MR_CREATE and KVM_MR_MOVE, and returns early for every other
> change. But the generic code allocates a zeroed memslot for every
> change and never copies old->arch, so after a KVM_MR_FLAGS_ONLY update,
> e.g. toggling KVM_MEM_LOG_DIRTY_PAGES for live migration, the active
> memslot has arch.flags == 0.
>
> With both flags clear, fault_supports_huge_mapping() falls through to
> the alignment check on the HVA range alone, which no longer verifies
> that the GPA and HVA have the same offset within a PMD. A memslot that
> was marked KVM_MEM_HUGEPAGE_INCAPABLE because of a GPA/HVA offset
> mismatch can then be mapped with PMD entries on read faults, and since
> kvm_map_page() aligns the gfn and the pfn independently, the guest ends
> up accessing the wrong host pages, exactly the "d -> f, e -> g" case
> described in the comment above the check.
>
> Carry the arch flags over from the old memslot for KVM_MR_FLAGS_ONLY,
> as the GPA, HVA and size are guaranteed to be unchanged for that case.
>
> Fixes: 7ab6fb505b2a ("LoongArch: KVM: Optimization for memslot hugepage checking")
> Cc: stable@vger.kernel.org
> Signed-off-by: Zeng Chi <zengchi@kylinos.cn>
> ---
> arch/loongarch/kvm/mmu.c | 10 ++++++++++
> 1 file changed, 10 insertions(+)
>
> diff --git a/arch/loongarch/kvm/mmu.c b/arch/loongarch/kvm/mmu.c
> index e104897aa532..4ca7c6ea8f08 100644
> --- a/arch/loongarch/kvm/mmu.c
> +++ b/arch/loongarch/kvm/mmu.c
> @@ -383,6 +383,16 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm, const struct kvm_memory_slot
> hva_t hva_start;
> size_t size, gpa_offset, hva_offset;
>
> + /*
> + * The generic code allocates a fresh, zeroed memslot for every change,
> + * so the arch flags computed below must be carried over when only the
> + * userspace flags change, e.g. when dirty logging is toggled.
> + */
> + if (change == KVM_MR_FLAGS_ONLY) {
> + new->arch = old->arch;
> + return 0;
> + }
> +
> if ((change != KVM_MR_MOVE) && (change != KVM_MR_CREATE))
> return 0;
> /*
> --
> 2.25.1
>
>
> No virus found
> Checked by Hillstone Network AntiVirus
>
^ permalink raw reply [flat|nested] 4+ messages in thread