linux-kernel - Re: [PATCH v2 07/14] x86/clear_page: add clear_page

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Message-ID: <87czm5ulcc.fsf@oracle.com>
Date:   Thu, 09 Dec 2021 20:37:07 -0800
From:   Ankur Arora <ankur.a.arora@...cle.com>
To:     Yu Xu <xuyu@...ux.alibaba.com>
Cc:     Ankur Arora <ankur.a.arora@...cle.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org, x86@...nel.org,
        mingo@...nel.org, bp@...en8.de, luto@...nel.org,
        akpm@...ux-foundation.org, mike.kravetz@...cle.com,
        jon.grimm@....com, kvm@...r.kernel.org, konrad.wilk@...cle.com,
        boris.ostrovsky@...cle.com
Subject: Re: [PATCH v2 07/14] x86/clear_page: add clear_page_uncached()


Yu Xu <xuyu@...ux.alibaba.com> writes:

> 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@...cle.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?
>
Hi Yu Xu,

Defined in include/linux/mm.h. Just below :).

>> +/*
>> + * 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);
>> +}

That said, as this note in the patch mentions, this isn't really a great
place for this definition. As you also mention, the right place for this
would be somewhere in the arch/.../include and include/asm-generic hierarchy.

>>      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.

Recommendations on how to handle this, welcome.

Thanks

--
ankur