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

[Xiao Guangrong <guangrong.xiao@gmail.com>]

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@intel.com>
> Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escareno@intel.com>
> Reported-by: Kumar, Karthik <karthik.kumar@intel.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?

It would be better if we move pat_enabled out of kvm as well, please refer
to pgprot_writecombine() which is implemented in pat.c and in
include\asm-generic\pgtable.h:

#ifndef pgprot_writecombine
#define pgprot_writecombine pgprot_noncached
#endif