linux-kernel - Re: [PATCH v4 3/3] KVM: MMU: consider host cache mode in MMIO page check

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Date:   Fri, 3 Nov 2017 17:24:43 +0800
From:   Xiao Guangrong <xiaoguangrong.eric@...il.com>
To:     Xiao Guangrong <guangrong.xiao@...il.com>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>, rkrcmar@...hat.com,
        Dan Williams <dan.j.williams@...el.com>,
        ivan.d.cuevas.escareno@...el.com, karthik.kumar@...el.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Olif Chapman <olif.chapman@...cle.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v4 3/3] KVM: MMU: consider host cache mode in MMIO page
 check



On 11/03/2017 05:02 PM, Haozhong Zhang wrote:
> On 11/03/17 16:51 +0800, Haozhong Zhang wrote:
>> On 11/03/17 14:54 +0800, Xiao Guangrong wrote:
>>>
>>>
>>> On 11/03/2017 01:53 PM, Haozhong Zhang wrote:
>>>> Some reserved pages, such as those from NVDIMM DAX devices, are
>>>> not for MMIO, and can be mapped with cached memory type for better
>>>> performance. However, the above check misconceives those pages as
>>>> MMIO.  Because KVM maps MMIO pages with UC memory type, the
>>>> performance of guest accesses to those pages would be harmed.
>>>> Therefore, we check the host memory type by lookup_memtype() in
>>>> addition and only treat UC/UC- pages as MMIO.
>>>>
>>>> Signed-off-by: Haozhong Zhang <haozhong.zhang@...el.com>
>>>> Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escareno@...el.com>
>>>> Reported-by: Kumar, Karthik <karthik.kumar@...el.com>
>>>> ---
>>>>    arch/x86/kvm/mmu.c | 19 ++++++++++++++++++-
>>>>    1 file changed, 18 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
>>>> index 0b481cc9c725..e9ed0e666a83 100644
>>>> --- a/arch/x86/kvm/mmu.c
>>>> +++ b/arch/x86/kvm/mmu.c
>>>> @@ -2708,7 +2708,24 @@ static bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
>>>>    static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
>>>>    {
>>>>    	if (pfn_valid(pfn))
>>>> -		return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn));
>>>> +		return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn)) &&
>>>> +			/*
>>>> +			 * Some reserved pages, such as those from
>>>> +			 * NVDIMM DAX devices, are not for MMIO, and
>>>> +			 * can be mapped with cached memory type for
>>>> +			 * better performance. However, the above
>>>> +			 * check misconceives those pages as MMIO.
>>>> +			 * Because KVM maps MMIO pages with UC memory
>>>> +			 * type, the performance of guest accesses to
>>>> +			 * those pages would be harmed. Therefore, we
>>>> +			 * check the host memory type in addition and
>>>> +			 * only treat UC/UC- pages as MMIO.
>>>> +			 *
>>>> +			 * pat_pfn_is_uc() works only when PAT is enabled,
>>>> +			 * so check pat_enabled() as well.
>>>> +			 */
>>>> +			(!pat_enabled() ||
>>>> +			 pat_pfn_is_uc(kvm_pfn_t_to_pfn_t(pfn)));
>>>
>>> Can it be compiled if !CONFIG_PAT?
>>
>> Yes.
>>
>> What I check via pat_enabled() is not only whether PAT support is
>> compiled, but also whether PAT is enabled at runtime.
>>
>>>
>>> It would be better if we move pat_enabled out of kvm as well,
>>
>> Surely I can combine them in one function like
>>
>> bool pat_pfn_is_uc(pfn_t pfn)
>> {
>> 	enum page_cache_mode cm;
>>
>> 	if (!pat_enabled())
>> 		return false;
>>
>> 	cm = lookup_memtype(pfn_t_to_phys(pfn));
>>
>> 	return cm == _PAGE_CACHE_MODE_UC || cm == _PAGE_CACHE_MODE_UC_MINUS;
>> }
> 
> In addition, I think it's better to split this function into
> pat_pfn_is_uc() and pat_pfn_is_uc_minus() to avoid additional
> confusion.

Why not use pat_pfn_is_uc_or_uc_minus(). :)