Re: [PATCH v2 07/14] x86/clear_page: add clear_page_uncached()

From: Yu Xu <xuyu@linux.alibaba.com>
To: Ankur Arora <ankur.a.arora@oracle.com>,
	linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org
Cc: mingo@kernel.org, bp@alien8.de, luto@kernel.org,
	akpm@linux-foundation.org, mike.kravetz@oracle.com,
	jon.grimm@amd.com, kvm@vger.kernel.org, konrad.wilk@oracle.com,
	boris.ostrovsky@oracle.com
Subject: Re: [PATCH v2 07/14] x86/clear_page: add clear_page_uncached()
Date: Wed, 8 Dec 2021 16:58:30 +0800	[thread overview]
Message-ID: <b955c5c4-bc4b-9f43-be1c-3a45973de259@linux.alibaba.com> (raw)
In-Reply-To: <20211020170305.376118-8-ankur.a.arora@oracle.com>

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