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SubjectRe: [PATCH v2 07/14] x86/clear_page: add clear_page_uncached()
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On 10/21/21 1:02 AM, Ankur Arora wrote:
> Expose the low-level uncached primitives (clear_page_movnt(),
> clear_page_clzero()) as alternatives via clear_page_uncached().
> Also fallback to clear_page(), if X86_FEATURE_MOVNT_SLOW is set
> and the CPU does not have X86_FEATURE_CLZERO.
>
> Both the uncached primitives use stores which are weakly ordered
> with respect to other instructions accessing the memory hierarchy.
> To ensure that callers don't mix accesses to different types of
> address_spaces, annotate clear_user_page_uncached(), and
> clear_page_uncached() as taking __incoherent pointers as arguments.
>
> Also add clear_page_uncached_make_coherent() which provides the
> necessary store fence to flush out the uncached regions.
>
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> ---
>
> Notes:
> This patch adds the fallback definitions of clear_user_page_uncached()
> etc in include/linux/mm.h which is likely not the right place for it.
>
> I'm guessing these should be moved to include/asm-generic/page.h
> (or maybe a new include/asm-generic/page_uncached.h) and for
> architectures that do have arch/$arch/include/asm/page.h (which
> seems like all of them), also replicate there?
>
> Anyway, wanted to first check if that's the way to do it, before
> doing that.
>
> arch/x86/include/asm/page.h | 10 ++++++++++
> arch/x86/include/asm/page_32.h | 9 +++++++++
> arch/x86/include/asm/page_64.h | 32 ++++++++++++++++++++++++++++++++
> include/linux/mm.h | 14 ++++++++++++++
> 4 files changed, 65 insertions(+)
>
> diff --git a/arch/x86/include/asm/page_32.h b/arch/x86/include/asm/page_32.h
> index 94dbd51df58f..163be03ac422 100644
> --- a/arch/x86/include/asm/page_32.h
> +++ b/arch/x86/include/asm/page_32.h
> @@ -39,6 +39,15 @@ static inline void clear_page(void *page)
> memset(page, 0, PAGE_SIZE);
> }
>
> +static inline void clear_page_uncached(__incoherent void *page)
> +{
> + clear_page((__force void *) page);
> +}
> +
> +static inline void clear_page_uncached_make_coherent(void)
> +{
> +}
> +
> static inline void copy_page(void *to, void *from)
> {
> memcpy(to, from, PAGE_SIZE);
> diff --git a/arch/x86/include/asm/page_64.h b/arch/x86/include/asm/page_64.h
> index 3c53f8ef8818..d7946047c70f 100644
> --- a/arch/x86/include/asm/page_64.h
> +++ b/arch/x86/include/asm/page_64.h
> @@ -56,6 +56,38 @@ static inline void clear_page(void *page)
> : "cc", "memory", "rax", "rcx");
> }
>
> +/*
> + * clear_page_uncached: only allowed on __incoherent memory regions.
> + */
> +static inline void clear_page_uncached(__incoherent void *page)
> +{
> + alternative_call_2(clear_page_movnt,
> + clear_page, X86_FEATURE_MOVNT_SLOW,
> + clear_page_clzero, X86_FEATURE_CLZERO,
> + "=D" (page),
> + "0" (page)
> + : "cc", "memory", "rax", "rcx");
> +}
> +
> +/*
> + * clear_page_uncached_make_coherent: executes the necessary store
> + * fence after which __incoherent regions can be safely accessed.
> + */
> +static inline void clear_page_uncached_make_coherent(void)
> +{
> + /*
> + * Keep the sfence for oldinstr and clzero separate to guard against
> + * the possibility that a cpu-model both has X86_FEATURE_MOVNT_SLOW
> + * and X86_FEATURE_CLZERO.
> + *
> + * The alternatives need to be in the same order as the ones
> + * in clear_page_uncached().
> + */
> + alternative_2("sfence",
> + "", X86_FEATURE_MOVNT_SLOW,
> + "sfence", X86_FEATURE_CLZERO);
> +}
> +
> void copy_page(void *to, void *from);
>
> #ifdef CONFIG_X86_5LEVEL
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 73a52aba448f..b88069d1116c 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -3192,6 +3192,20 @@ static inline bool vma_is_special_huge(const struct vm_area_struct *vma)
>
> #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
>
> +#ifndef clear_user_page_uncached

Hi Ankur Arora,

I've been looking for where clear_user_page_uncached is defined in this
patchset, but failed.

There should be something like follows in arch/x86, right?

static inline void clear_user_page_uncached(__incoherent void *page,
unsigned long vaddr, struct page *pg)
{
clear_page_uncached(page);
}


Did I miss something?

> +/*
> + * clear_user_page_uncached: fallback to the standard clear_user_page().
> + */
> +static inline void clear_user_page_uncached(__incoherent void *page,
> + unsigned long vaddr, struct page *pg)
> +{
> + clear_user_page((__force void *)page, vaddr, pg);
> +}
> +
> +static inline void clear_page_uncached_make_coherent(void) { }
> +#endif
> +
> +
> #ifdef CONFIG_DEBUG_PAGEALLOC
> extern unsigned int _debug_guardpage_minorder;
> DECLARE_STATIC_KEY_FALSE(_debug_guardpage_enabled);
>

--
Thanks,
Yu

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