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

["Raslan, KarimAllah" <karahmed@amazon.de>]

On Fri, 2018-12-21 at 16:20 +0100, Paolo Bonzini wrote:
> On 06/12/18 00:10, Jim Mattson wrote:
> > 
> > On Mon, Dec 3, 2018 at 1:31 AM KarimAllah Ahmed <karahmed@amazon.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@amazon.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.

I can not use "cached_vmcs12" directly here because its old contents will be 
used for "nested_release_vmcs12(..)" a few lines below (i.e. it will be flushed 
into guest memory).

I can just switch this to the new API introduced a few patches later and that 
would have the same semantics for normal use-cases as before.

> 
> Paolo



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