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

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Message-ID: <8a2a3b3d-b16c-890c-e6cd-69fbfc06506a@redhat.com>
Date:   Fri, 21 Dec 2018 16:20:12 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>,
        KarimAllah Ahmed <karahmed@...zon.de>
Cc:     Radim Krčmář <rkrcmar@...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 06/12/18 00:10, Jim Mattson wrote:
> 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?

It wouldn't be slower if he read directly into cached_vmcs12.  However,
as it is now, it's doing two reads instead of one.  By doing this, the
ENOMEM case also disappears.

Paolo