Re: [PATCH 1/1 v2] KVM: MMU: Optimize guest page table walk

[Jan Kiszka <jan.kiszka@web.de>]

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On 2011-04-21 17:34, Takuya Yoshikawa wrote:
> From: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
> 
> This patch optimizes the guest page table walk by using get_user()
> instead of copy_from_user().
> 
> With this patch applied, paging64_walk_addr_generic() has become
> about 0.5us to 1.0us faster on my Phenom II machine with NPT on.
> 
> Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
> ---
>  arch/x86/kvm/paging_tmpl.h |   23 ++++++++++++++++++++---
>  1 files changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index 74f8567..825d953 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -117,6 +117,7 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
>  				    gva_t addr, u32 access)
>  {
>  	pt_element_t pte;
> +	pt_element_t __user *ptep_user;
>  	gfn_t table_gfn;
>  	unsigned index, pt_access, uninitialized_var(pte_access);
>  	gpa_t pte_gpa;
> @@ -152,6 +153,9 @@ walk:
>  	pt_access = ACC_ALL;
>  
>  	for (;;) {
> +		gfn_t real_gfn;
> +		unsigned long host_addr;
> +
>  		index = PT_INDEX(addr, walker->level);
>  
>  		table_gfn = gpte_to_gfn(pte);
> @@ -160,9 +164,22 @@ walk:
>  		walker->table_gfn[walker->level - 1] = table_gfn;
>  		walker->pte_gpa[walker->level - 1] = pte_gpa;
>  
> -		if (kvm_read_guest_page_mmu(vcpu, mmu, table_gfn, &pte,
> -					    offset, sizeof(pte),
> -					    PFERR_USER_MASK|PFERR_WRITE_MASK)) {
> +		real_gfn = mmu->translate_gpa(vcpu, gfn_to_gpa(table_gfn),
> +					      PFERR_USER_MASK|PFERR_WRITE_MASK);
> +		if (real_gfn == UNMAPPED_GVA) {
> +			present = false;
> +			break;
> +		}
> +		real_gfn = gpa_to_gfn(real_gfn);
> +
> +		host_addr = gfn_to_hva(vcpu->kvm, real_gfn);
> +		if (kvm_is_error_hva(host_addr)) {
> +			present = false;
> +			break;
> +		}
> +
> +		ptep_user = (pt_element_t __user *)((void *)host_addr + offset);
> +		if (get_user(pte, ptep_user)) {
                    ^^^^^^^^^^^^
This doesn't work for x86-32: pte is 64 bit, but get_user is only
defined up to 32 bit on that platform.

Avi, what's your 32-bit buildbot doing? :)

Jan


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