linux-kernel - Re: [PATCH v2] KVM: x86/pt: Ignore all unknown Intel PT capabilities

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Date:   Tue, 11 Jan 2022 15:59:43 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Like Xu <like.xu.linux@...il.com>,
        Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Joerg Roedel <joro@...tes.org>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86/pt: Ignore all unknown Intel PT capabilities

On 1/11/2022 12:20 PM, Like Xu wrote:
> On 11/1/2022 8:57 am, Sean Christopherson wrote:
>> On Mon, Jan 10, 2022, Like Xu wrote:
>>> From: Like Xu <likexu@...cent.com>
>>>
>>> Some of the new Intel PT capabilities (e.g. SDM Vol3, 32.2.4 Event
>>> Tracing, it exposes details about the asynchronous events, when they are
>>> generated, and when their corresponding software event handler completes
>>> execution) cannot be safely and fully emulated by the KVM, especially
>>> emulating the simultaneous writing of guest PT packets generated by
>>> the KVM to the guest PT buffer.
>>>
>>> For KVM, it's better to advertise currently supported features based on
>>> the "static struct pt_cap_desc" implemented in the host PT driver and
>>> ignore _all_ unknown features before they have been investigated one by
>>> one and supported in a safe manner, leaving the rest as system-wide-only
>>> tracing capabilities.
>>>
>>> Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
>>> Signed-off-by: Like Xu <likexu@...cent.com>
>>> ---
>>> v1 -> v2 Changelog:
>>> - Be safe and ignore _all_ unknown capabilities. (Paolo)
>>>
>>> Previous:
>>> https://lore.kernel.org/kvm/20220106085533.84356-1-likexu@tencent.com/
>>>
>>>   arch/x86/kvm/cpuid.c | 2 ++
>>>   1 file changed, 2 insertions(+)
>>>
>>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>>> index 0b920e12bb6d..439b93359848 100644
>>> --- a/arch/x86/kvm/cpuid.c
>>> +++ b/arch/x86/kvm/cpuid.c
>>> @@ -901,6 +901,8 @@ static inline int __do_cpuid_func(struct 
>>> kvm_cpuid_array *array, u32 function)
>>>               break;
>>>           }
>>> +        /* It's better to be safe and ignore _all_ unknown 
>>> capabilities. */
>>
>> No need to justify why unknown capabilities are hidden as that's very 
>> much (supposed
>> to be) standard KVM behavior.
>>
>>> +        entry->ebx &= GENMASK(5, 0);
>>
>> Please add a #define somewhere so that this is self-documenting, e.g. see
>> KVM_SUPPORTED_XCR0.
> 
> How about we define this macro in the <asm/intel_pt.h> so that the next 
> PT capability
> enabler can update the mask with minimal effort, considering that many 
> pure kernel
> developers don't care about KVM code ?
> 
>>
>> And why just EBX?  ECX appears to enumerate features too, and EDX is 
>> presumably
>> reserved to enumerate yet more features when EBX/ECX run out of bits.
> 
> Yes, how about this version:
> 
> diff --git a/arch/x86/include/asm/intel_pt.h 
> b/arch/x86/include/asm/intel_pt.h
> index ebe8d2ea44fe..da94d0eeb9df 100644
> --- a/arch/x86/include/asm/intel_pt.h
> +++ b/arch/x86/include/asm/intel_pt.h
> @@ -24,6 +24,12 @@ enum pt_capabilities {
>       PT_CAP_psb_periods,
>   };
> 
> +#define GUEST_SUPPORTED_CPUID_14_EBX    \
> +    (BIT(0) | BIT(1) | BIT(2) | BIT(3) | BIT(4) | BIT(5))
> +
> +#define GUEST_SUPPORTED_CPUID_14_ECX    \
> +    (BIT(0) | BIT(1) | BIT(2) | BIT(3) | BIT(31))
> +

I doubt BIT(3) of CPUID_14_ECX can be exposed to guest directly.

It means "output to Trace Transport Subsystem Supported". If I 
understand correctly, it at least needs passthrough of the said 
Transport Subsystem or emulation of it.

>   #if defined(CONFIG_PERF_EVENTS) && defined(CONFIG_CPU_SUP_INTEL)
>   void cpu_emergency_stop_pt(void);
>   extern u32 intel_pt_validate_hw_cap(enum pt_capabilities cap);
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 0b920e12bb6d..be8c9170f98e 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -19,6 +19,7 @@
>   #include <asm/user.h>
>   #include <asm/fpu/xstate.h>
>   #include <asm/sgx.h>
> +#include <asm/intel_pt.h>
>   #include "cpuid.h"
>   #include "lapic.h"
>   #include "mmu.h"
> @@ -900,7 +901,10 @@ static inline int __do_cpuid_func(struct 
> kvm_cpuid_array *array, u32 function)
>               entry->eax = entry->ebx = entry->ecx = entry->edx = 0;
>               break;
>           }
> -
> +        entry->eax = min(entry->eax, 1u);
> +        entry->ebx &= GUEST_SUPPORTED_CPUID_14_EBX;
> +        entry->ecx &= GUEST_SUPPORTED_CPUID_14_ECX;
> +        entry->edx = 0;
>           for (i = 1, max_idx = entry->eax; i <= max_idx; ++i) {
>               if (!do_host_cpuid(array, function, i))
>                   goto out;