Re: [PATCH v2 RFC] x86/kvm/mmu: make mmu->prev_roots cache work for NPT case

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Vitaly Kuznetsov <vkuznets@redhat.com> writes:

> Alternative patch: remove the filtering from kvm_mmu_get_page() and check
> for direct on call sites. cr4_pae setting in kvm_calc_mmu_role_common()
> can be preserved for consistency.
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  arch/x86/kvm/mmu.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index f2d1d230d5b8..7fb8118f2af6 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2420,8 +2420,6 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu,
>  	role = vcpu->arch.mmu->mmu_role.base;
>  	role.level = level;
>  	role.direct = direct;
> -	if (role.direct)
> -		role.cr4_pae = 0;
>  	role.access = access;
>  	if (!vcpu->arch.mmu->direct_map
>  	    && vcpu->arch.mmu->root_level <= PT32_ROOT_LEVEL) {
> @@ -5176,7 +5174,7 @@ static bool detect_write_misaligned(struct kvm_mmu_page *sp, gpa_t gpa,
>  		 gpa, bytes, sp->role.word);
>  
>  	offset = offset_in_page(gpa);
> -	pte_size = sp->role.cr4_pae ? 8 : 4;
> +	pte_size = (sp->role.cr4_pae && !sp->role.direct) ? 8 : 4;
>  
>  	/*
>  	 * Sometimes, the OS only writes the last one bytes to update status
> @@ -5200,7 +5198,7 @@ static u64 *get_written_sptes(struct kvm_mmu_page *sp, gpa_t gpa, int *nspte)
>  	page_offset = offset_in_page(gpa);
>  	level = sp->role.level;
>  	*nspte = 1;
> -	if (!sp->role.cr4_pae) {
> +	if (!sp->role.cr4_pae || sp->role.direct) {
>  		page_offset <<= 1;	/* 32->64 */
>  		/*
>  		 * A 32-bit pde maps 4MB while the shadow pdes map

While I personally prefer this approach to not setting role.cr4_pae in
kvm_calc_mmu_role_common() I'd like to get maintainers opinion (I did
test the patch with ept=off but I have to admit I don't know much about
shadow page tables which actually use detect_write_misaligned()/
get_written_sptes())

Paolo, Radim, (anyone else) - any thoughts?

-- 
Vitaly