Re: [RFC PATCH v2 02/11] KVM: x86: Advertise CPUID.7.2.EDX and RRSBA_CTRL support

[Xiaoyao Li]

On 5/16/2023 10:04 AM, Chao Gao wrote:
> On Mon, May 15, 2023 at 02:53:07PM +0800, Xiaoyao Li wrote:
> > On 4/14/2023 2:25 PM, Chao Gao wrote:
> > > From: Zhang Chen <chen.zhang@xxxxxxxxx>
> > >
> > > Add a kvm-only CPUID feature leaf for CPUID.7.2.EDX and RRSBA_CTRL
> > > as the first feature in the leaf.
> > >
> > > RRSBA_CTRL is enumerated by CPUID.7.2.EDX[2]. If supported, RRSBA_DIS_U
> > > (bit 5) and RRSBA_DIS_S (bit 6) of IA32_SPEC_CTRL MSR can be used to
> > > disable RRSBA behavior for CPL3 and CPL0/1/2 respectively.
> > >
> > > Note that KVM does not intercept guests' IA32_SPEC_CTRL MSR accesses
> > > after a non-zero is written to the MSR. Therefore, guests can already
> > > toggle the two bits if the host supports RRSBA_CTRL, and no extra code
> > > is needed to allow guests to toggle the two bits.
> >
> > This is a bug that also matters with other bits in MSR_IA32_SPEC_CTRL which
> > has a dedicated enumeration CPUID bit and no support in KVM yet.
>
> Do you mean passing through the MSR is a bug? guest can write any hardware
> supported value to the MSR if the MSR isn't intercepted.
>
> I guess this is intentional and a trade-off for performance (note that
> context-switch may cause writes to the MSR). And see
>
> commit 841c2be09fe4 ("kvm: x86: replace kvm_spec_ctrl_test_value with runtime test on the host")
>
> it appears that this behavior is widely recognized.
>
> > I think we need to fix this bug at first.
>
> I have no idea how to fix the "bug" without intercepting the MSR. The
> performance penalty makes me think intercepting the MSR is not a viable
> solution.

I thought correctness always takes higher priority over performance.

> > > Signed-off-by: Zhang Chen <chen.zhang@xxxxxxxxx>
> > > Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> > > Tested-by: Jiaan Lu <jiaan.lu@xxxxxxxxx>
> > > ---
> > >    arch/x86/kvm/cpuid.c         | 22 +++++++++++++++++++---
> > >    arch/x86/kvm/reverse_cpuid.h |  7 +++++++
> > >    2 files changed, 26 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > index 9583a110cf5f..f024c3ac2203 100644
> > > --- a/arch/x86/kvm/cpuid.c
> > > +++ b/arch/x86/kvm/cpuid.c
> > > @@ -685,6 +685,10 @@ void kvm_set_cpu_caps(void)
> > >    		SF(SGX1) | SF(SGX2) | SF(SGX_EDECCSSA)
> > >    	);
> > > +	kvm_cpu_cap_init_kvm_defined(CPUID_7_2_EDX,
> > > +		SF(RRSBA_CTRL)
> > > +	);
> > > +
> >
> > Please move this hook up to right follow the leaf CPUID_7_1_EAX.
>
> sure. will do.