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Message-ID: <CAEvNRgGXeow48BUJYyuAOUp8qK97v1LdF4KdTB=Nbk7pTs9tfw@mail.gmail.com>
Date: Thu, 15 Jan 2026 13:07:05 -0800
From: Ackerley Tng <ackerleytng@...gle.com>
To: "Kalyazin, Nikita" <kalyazin@...zon.co.uk>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
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Subject: Re: [PATCH v9 01/13] set_memory: add folio_{zap,restore}_direct_map helpers
"Kalyazin, Nikita" <kalyazin@...zon.co.uk> writes:
> From: Nikita Kalyazin <kalyazin@...zon.com>
>
> These allow guest_memfd to remove its memory from the direct map.
> Only implement them for architectures that have direct map.
> In folio_zap_direct_map(), flush TLB on architectures where
> set_direct_map_valid_noflush() does not flush it internally.
>
> The new helpers need to be accessible to KVM on architectures that
> support guest_memfd (x86 and arm64). Since arm64 does not support
> building KVM as a module, only export them on x86.
>
> Direct map removal gives guest_memfd the same protection that
> memfd_secret does, such as hardening against Spectre-like attacks
> through in-kernel gadgets.
>
> Signed-off-by: Nikita Kalyazin <kalyazin@...zon.com>
> ---
> arch/arm64/include/asm/set_memory.h | 2 ++
> arch/arm64/mm/pageattr.c | 12 ++++++++++++
> arch/loongarch/include/asm/set_memory.h | 2 ++
> arch/loongarch/mm/pageattr.c | 16 ++++++++++++++++
> arch/riscv/include/asm/set_memory.h | 2 ++
> arch/riscv/mm/pageattr.c | 16 ++++++++++++++++
> arch/s390/include/asm/set_memory.h | 2 ++
> arch/s390/mm/pageattr.c | 18 ++++++++++++++++++
> arch/x86/include/asm/set_memory.h | 2 ++
> arch/x86/mm/pat/set_memory.c | 20 ++++++++++++++++++++
> include/linux/set_memory.h | 10 ++++++++++
> 11 files changed, 102 insertions(+)
>
> diff --git a/arch/arm64/include/asm/set_memory.h b/arch/arm64/include/asm/set_memory.h
> index 90f61b17275e..d949f1deb701 100644
> --- a/arch/arm64/include/asm/set_memory.h
> +++ b/arch/arm64/include/asm/set_memory.h
> @@ -14,6 +14,8 @@ int set_memory_valid(unsigned long addr, int numpages, int enable);
> int set_direct_map_invalid_noflush(struct page *page);
> int set_direct_map_default_noflush(struct page *page);
> int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid);
> +int folio_zap_direct_map(struct folio *folio);
> +int folio_restore_direct_map(struct folio *folio);
> bool kernel_page_present(struct page *page);
>
> int set_memory_encrypted(unsigned long addr, int numpages);
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index f0e784b963e6..a94eff324dda 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -357,6 +357,18 @@ int set_direct_map_valid_noflush(struct page *page, unsigned nr, bool valid)
> return set_memory_valid(addr, nr, valid);
> }
>
> +int folio_zap_direct_map(struct folio *folio)
> +{
> + return set_direct_map_valid_noflush(folio_page(folio, 0),
> + folio_nr_pages(folio), false);
> +}
> +
> +int folio_restore_direct_map(struct folio *folio)
> +{
> + return set_direct_map_valid_noflush(folio_page(folio, 0),
> + folio_nr_pages(folio), true);
> +}
> +
Was going to suggest a _noflush suffix to these functions, but saw
Aneesh's comment that these functions actually do flush_tlb_kernel [1]
[1] https://lore.kernel.org/all/yq5ajz07czvz.fsf@kernel.org/
Reviewed-by: Ackerley Tng <ackerleytng@...gle.com>
> #ifdef CONFIG_DEBUG_PAGEALLOC
> /*
> * This is - apart from the return value - doing the same
>
> [...snip...]
>
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