On Thu, Dec 6, 2018 at 9:02 AM Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Wed, Dec 05, 2018 at 03:28:59PM -0800, Jim Mattson wrote:
> > Add a comment explaining why MSR_STAR must be included in
> > vmx_msr_index[] even for i386 builds.
> >
> > The elided comment has not been relevant since move_msr_up() was
> > introduced in commit a75beee6e4f5d ("KVM: VMX: Avoid saving and
> > restoring msrs on lightweight vmexit").
> >
> > Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx>
> > Reviewed-by: Peter Shier <pshier@xxxxxxxxxx>
> > ---
> > arch/x86/kvm/vmx.c | 7 +++++--
> > 1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> > index ad1753e8a85e5..b58c3952c5dfb 100644
> > --- a/arch/x86/kvm/vmx.c
> > +++ b/arch/x86/kvm/vmx.c
> > @@ -1390,8 +1390,11 @@ static u64 host_efer;
> > static void ept_save_pdptrs(struct kvm_vcpu *vcpu);
> >
> > /*
> > - * Keep MSR_STAR at the end, as setup_msrs() will try to optimize it
> > - * away by decrementing the array size.
> > + * Though SYSCALL is only supported in 64-bit mode on Intel CPUs, kvm
> > + * will emulate SYSCALL in legacy mode if the vendor string in guest
> > + * CPUID.0:{EBX,ECX,EDX} is "AuthenticAMD" or "AMDisbetter!" To
> > + * support this emulation, IA32_STAR must always be included in
> > + * vmx_msr_index[], even in i386 builds.
> > */
>
> Hmm, so this isn't wrong per se, but relying on the detection of hardware
> MSRs for software emulation is weird, e.g. vmx_vcpu_setup() will only
> "find" MSR_STAR if it exists in hardware even though an i386 build will
> never actually touch the hardware MSR. This also applies to MSR_CSTAR in
> the next patch (for all builds).
>
> What about making vmx_msr_index an array of structs and adding a flag to
> denote an emulation-only MSR? That'd document why we keep certain MSRs
> around and would also avoid probing them in vmx_vcpu_setup(), e.g. I'm
> assuming MSR_EFER is kept in i386 build for emulation too.
The management of this array seems overly convoluted already. Let me
see if I can get the existing patch set accepted first, and then maybe
we can come up with something just a little less baroque.
