Re: [PATCH v2 4/4] KVM: x86: Expose Architectural LBR CPUID and its XSAVES bit

["Xu, Like" <like.xu@intel.com>]

Hi Paolo,

I am wondering if it is acceptable for you to
review the minor Architecture LBR patch set without XSAVES for v5.12 ?

As far as I know, the guest Arch LBR  can still work without XSAVES support.

---
thx,likexu

On 2021/2/4 8:59, Xu, Like wrote:
> On 2021/2/3 22:37, Paolo Bonzini wrote:
>> On 03/02/21 14:57, Like Xu wrote:
>>> If CPUID.(EAX=07H, ECX=0):EDX[19] is exposed to 1, the KVM supports Arch
>>> LBRs and CPUID leaf 01CH indicates details of the Arch LBRs capabilities.
>>> As the first step, KVM only exposes the current LBR depth on the host for
>>> guest, which is likely to be the maximum supported value on the host.
>>>
>>> If KVM supports XSAVES, the CPUID.(EAX=0DH, ECX=1):EDX:ECX[bit 15]
>>> is also exposed to 1, which means the availability of support for Arch
>>> LBR configuration state save and restore. When available, guest software
>>> operating at CPL=0 can use XSAVES/XRSTORS manage supervisor state
>>> component Arch LBR for own purposes once IA32_XSS [bit 15] is set.
>>> XSAVE support for Arch LBRs is enumerated in CPUID.(EAX=0DH, ECX=0FH).
>>>
>>> Signed-off-by: Like Xu <like.xu@linux.intel.com>
>>> ---
>>>   arch/x86/kvm/cpuid.c   | 23 +++++++++++++++++++++++
>>>   arch/x86/kvm/vmx/vmx.c |  2 ++
>>>   arch/x86/kvm/x86.c     | 10 +++++++++-
>>>   3 files changed, 34 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>>> index 944f518ca91b..900149eec42d 100644
>>> --- a/arch/x86/kvm/cpuid.c
>>> +++ b/arch/x86/kvm/cpuid.c
>>> @@ -778,6 +778,29 @@ static inline int __do_cpuid_func(struct 
>>> kvm_cpuid_array *array, u32 function)
>>>               entry->edx = 0;
>>>           }
>>>           break;
>>> +    /* Architectural LBR */
>>> +    case 0x1c:
>>> +    {
>>> +        u64 lbr_depth_mask = 0;
>>> +
>>> +        if (!kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR)) {
>>> +            entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
>>> +            break;
>>> +        }
>>> +
>>> +        /*
>>> +         * KVM only exposes the maximum supported depth,
>>> +         * which is also the fixed value used on the host.
>>> +         *
>>> +         * KVM doesn't allow VMM user sapce to adjust depth
>>> +         * per guest, because the guest LBR emulation depends
>>> +         * on the implementation of the host LBR driver.
>>> +         */
>>> +        lbr_depth_mask = 1UL << fls(entry->eax & 0xff);
>>> +        entry->eax &= ~0xff;
>>> +        entry->eax |= lbr_depth_mask;
>>> +        break;
>>> +    }
>>>       /* Intel PT */
>>>       case 0x14:
>>>           if (!kvm_cpu_cap_has(X86_FEATURE_INTEL_PT)) {
>>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>>> index 9ddf0a14d75c..c22175d9564e 100644
>>> --- a/arch/x86/kvm/vmx/vmx.c
>>> +++ b/arch/x86/kvm/vmx/vmx.c
>>> @@ -7498,6 +7498,8 @@ static __init void vmx_set_cpu_caps(void)
>>>           kvm_cpu_cap_check_and_set(X86_FEATURE_INVPCID);
>>>       if (vmx_pt_mode_is_host_guest())
>>>           kvm_cpu_cap_check_and_set(X86_FEATURE_INTEL_PT);
>>> +    if (cpu_has_vmx_arch_lbr())
>>> +        kvm_cpu_cap_check_and_set(X86_FEATURE_ARCH_LBR);
>>>         if (vmx_umip_emulated())
>>>           kvm_cpu_cap_set(X86_FEATURE_UMIP);
>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>> index 667d0042d0b7..107f2e72f526 100644
>>> --- a/arch/x86/kvm/x86.c
>>> +++ b/arch/x86/kvm/x86.c
>>> @@ -10385,8 +10385,16 @@ int kvm_arch_hardware_setup(void *opaque)
>>>         if (!kvm_cpu_cap_has(X86_FEATURE_XSAVES))
>>>           supported_xss = 0;
>>> -    else
>>> +    else {
>>>           supported_xss &= host_xss;
>>> +        /*
>>> +         * The host doesn't always set ARCH_LBR bit to hoss_xss since 
>>> this
>>> +         * Arch_LBR component is used on demand in the Arch LBR driver.
>>> +         * Check e649b3f0188f "Support dynamic supervisor feature for 
>>> LBR".
>>> +         */
>>> +        if (kvm_cpu_cap_has(X86_FEATURE_ARCH_LBR))
>>> +            supported_xss |= XFEATURE_MASK_LBR;
>>> +    }
>>>         /* Update CET features now that supported_xss is finalized. */
>>>       if (!kvm_cet_supported()) {
>>>
>>
>> This requires some of the XSS patches that Weijang posted for CET, right?
>
> Yes, at least we need three of them for Arch LBR:
>
> 3009dfd6d61f KVM: x86: Load guest fpu state when accessing MSRs managed 
> by XSAVES
> d39b0a16ad1f KVM: x86: Refresh CPUID on writes to MSR_IA32_XSS
> e98bf65e51c9 KVM: x86: Report XSS as an MSR to be saved if there are 
> supported features
>
>>
>> Also, who takes care of saving/restoring the MSRs, if the host has not 
>> added XFEATURE_MASK_LBR to MSR_IA32_XSS?
>
> I may not understand your concern on this. Let me try to explain:
>
> The guest Arch LBR driver will save the origin host_xss and
> mark the LBR bit only in the XSS and then save/restore MSRs
> in the extra specified guest memory, and restore the origin host_xss.
>
> On the host side, the same thing happens to vcpu thread
> due to the help of guest LBR event created by the vPMU
> and the hardware LBR MSRs are saved/restored in a exclusive way.
>
>>
>> Thanks,
>>
>> Paolo
>>
>