linux-kernel - Re: [PATCH v4 02/14] X86/nVMX: handle_vmptrld: Copy the VMCS12 directly from guest memory

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Date:   Wed, 5 Dec 2018 15:10:02 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     KarimAllah Ahmed <karahmed@...zon.de>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>
Subject: Re: [PATCH v4 02/14] X86/nVMX: handle_vmptrld: Copy the VMCS12
 directly from guest memory

On Mon, Dec 3, 2018 at 1:31 AM KarimAllah Ahmed <karahmed@...zon.de> wrote:
>
> Copy the VMCS12 directly from guest memory instead of the map->copy->unmap
> sequence. This also avoids using kvm_vcpu_gpa_to_page() and kmap() which
> assumes that there is a "struct page" for guest memory.
>
> Signed-off-by: KarimAllah Ahmed <karahmed@...zon.de>
> ---
> v3 -> v4:
> - Return VMXERR_VMPTRLD_INCORRECT_VMCS_REVISION_ID on failure (jmattson@)
> v1 -> v2:
> - Massage commit message a bit.
> ---
>  arch/x86/kvm/vmx.c | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index b84f230..75817cb 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -9301,20 +9301,22 @@ static int handle_vmptrld(struct kvm_vcpu *vcpu)
>                 return 1;
>
>         if (vmx->nested.current_vmptr != vmptr) {
> -               struct vmcs12 *new_vmcs12;
> -               struct page *page;
> -               page = kvm_vcpu_gpa_to_page(vcpu, vmptr);
> -               if (is_error_page(page))
> -                       return nested_vmx_failInvalid(vcpu);
> +               struct vmcs12 *new_vmcs12 = (struct vmcs12 *)__get_free_page(GFP_KERNEL);
> +
> +               if (!new_vmcs12 ||
> +                   kvm_read_guest(vcpu->kvm, vmptr, new_vmcs12,
> +                                  sizeof(*new_vmcs12))) {

Isn't this a lot slower than kmap() when there is a struct page?

> +                       free_page((unsigned long)new_vmcs12);
> +                       return nested_vmx_failValid(vcpu,
> +                                                   VMXERR_VMPTRLD_INCORRECT_VMCS_REVISION_ID);

I agree with David that this isn't the right thing to do for -ENOMEM.