Re: [PATCH v2 33/49] KVM: x86: Advertise TSC_DEADLINE_TIMER in KVM_GET_SUPPORTED_CPUID

[Maxim Levitsky]

On Fri, 2024-05-17 at 10:39 -0700, Sean Christopherson wrote:
> Advertise TSC_DEADLINE_TIMER via KVM_GET_SUPPORTED_CPUID when it's
> supported in hardware, as the odds of a VMM emulating the local APIC in
> userspace, not emulating the TSC deadline timer, _and_ reflecting
> KVM_GET_SUPPORTED_CPUID back into KVM_SET_CPUID2 are extremely low.
> 
> KVM has _unconditionally_ advertised X2APIC via CPUID since commit
> 0d1de2d901f4 ("KVM: Always report x2apic as supported feature"), and it
> is completely impossible for userspace to emulate X2APIC as KVM doesn't
> support forwarding the MSR accesses to userspace.  I.e. KVM has relied on
> userspace VMMs to not misreport local APIC capabilities for nearly 13
> years.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  Documentation/virt/kvm/api.rst | 9 ++++++---
>  arch/x86/kvm/cpuid.c           | 4 ++--
>  2 files changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 884846282d06..cb744a646de6 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -1804,15 +1804,18 @@ emulate them efficiently. The fields in each entry are defined as follows:
>           the values returned by the cpuid instruction for
>           this function/index combination
>  
> -The TSC deadline timer feature (CPUID leaf 1, ecx[24]) is always returned
> -as false, since the feature depends on KVM_CREATE_IRQCHIP for local APIC
> -support.  Instead it is reported via::
> +x2APIC (CPUID leaf 1, ecx[21) and TSC deadline timer (CPUID leaf 1, ecx[24])
> +may be returned as true, but they depend on KVM_CREATE_IRQCHIP for in-kernel
> +emulation of the local APIC.  TSC deadline timer support is also reported via::
>  
>    ioctl(KVM_CHECK_EXTENSION, KVM_CAP_TSC_DEADLINE_TIMER)
>  
>  if that returns true and you use KVM_CREATE_IRQCHIP, or if you emulate the
>  feature in userspace, then you can enable the feature for KVM_SET_CPUID2.
>  
> +Enabling x2APIC in KVM_SET_CPUID2 requires KVM_CREATE_IRQCHIP as KVM doesn't
> +support forwarding x2APIC MSR accesses to userspace, i.e. KVM does not support
> +emulating x2APIC in userspace.
>  
>  4.47 KVM_PPC_GET_PVINFO
>  -----------------------
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 699ce4261e9c..d1f427284ccc 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -680,8 +680,8 @@ void kvm_set_cpu_caps(void)
>  		F(FMA) | F(CX16) | 0 /* xTPR Update */ | F(PDCM) |
>  		F(PCID) | 0 /* Reserved, DCA */ | F(XMM4_1) |
>  		F(XMM4_2) | EMUL_F(X2APIC) | F(MOVBE) | F(POPCNT) |
> -		0 /* Reserved*/ | F(AES) | F(XSAVE) | 0 /* OSXSAVE */ | F(AVX) |
> -		F(F16C) | F(RDRAND)
> +		EMUL_F(TSC_DEADLINE_TIMER) | F(AES) | F(XSAVE) |
> +		0 /* OSXSAVE */ | F(AVX) | F(F16C) | F(RDRAND)
>  	);
>  
>  	kvm_cpu_cap_init(CPUID_1_EDX,

Hi,

I have a mixed feeling about this.

First of all KVM_GET_SUPPORTED_CPUID documentation explicitly states that it returns bits
that are supported in *default* configuration
TSC_DEADLINE_TIMER and arguably X2APIC are only supported after enabling various caps,
e.g not default configuration.

However, since X2APIC also in KVM_GET_SUPPORTED_CPUID (also wrongly IMHO), for consistency it does make
sense to add TSC_DEADLINE_TIMER as well.


I do think that we need at least to update the documentation of KVM_GET_SUPPORTED_CPUID
and KVM_GET_EMULATED_CPUID, as I state in a review of a later patch.

Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky