linux-kernel - Re: [PATCH v4 08/16] KVM: x86/pmu: Add IA32_DS_AREA MSR emulation to manage guest DS buffer

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Date:   Thu, 8 Apr 2021 13:39:49 +0800
From:   "Xu, Like" <like.xu@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>, eranian@...gle.com,
        andi@...stfloor.org, kan.liang@...ux.intel.com,
        wei.w.wang@...el.com, Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Andi Kleen <ak@...ux.intel.com>,
        Like Xu <like.xu@...ux.intel.com>
Subject: Re: [PATCH v4 08/16] KVM: x86/pmu: Add IA32_DS_AREA MSR emulation to
 manage guest DS buffer

Hi Peter,

Thanks for your detailed comments.

If you have more comments for other patches, please let me know.

On 2021/4/7 23:39, Peter Zijlstra wrote:
> On Mon, Mar 29, 2021 at 01:41:29PM +0800, Like Xu wrote:
>> @@ -3869,10 +3876,12 @@ static struct perf_guest_switch_msr *intel_guest_get_msrs(int *nr, void *data)
>>   
>>   		if (arr[1].guest)
>>   			arr[0].guest |= arr[1].guest;
>> -		else
>> +		else {
>>   			arr[1].guest = arr[1].host;
>> +			arr[2].guest = arr[2].host;
>> +		}
> What's all this gibberish?
>
> The way I read that it says:
>
> 	if guest has PEBS_ENABLED
> 		guest GLOBAL_CTRL |= PEBS_ENABLED
> 	otherwise
> 		guest PEBS_ENABLED = host PEBS_ENABLED
> 		guest DS_AREA = host DS_AREA
>
> which is just completely random garbage afaict. Why would you leak host
> msrs into the guest?

In fact, this is not a leak at all.

When we do "arr[i].guest = arr[i].host;" assignment in the 
intel_guest_get_msrs(),
the KVM will check "if (msrs[i].host == msrs[i].guest)" and if so, it 
disables the atomic
switch for this msr during vmx transaction in the caller 
atomic_switch_perf_msrs().

In that case, the msr value doesn't change and any guest write will be trapped.
If the next check is "msrs[i].host != msrs[i].guest", the atomic switch 
will be triggered again.

Compared to before, this part of the logic has not changed, which helps to 
reduce overhead.

> Why would you change guest GLOBAL_CTRL implicitly;

This is because in the early part of this function, we have operations:

     if (x86_pmu.flags & PMU_FL_PEBS_ALL)
         arr[0].guest &= ~cpuc->pebs_enabled;
     else
         arr[0].guest &= ~(cpuc->pebs_enabled & PEBS_COUNTER_MASK);

and if guest has PEBS_ENABLED, we need these bits back for PEBS counters:

     arr[0].guest |= arr[1].guest;

> guest had better wrmsr that himself to control when stuff is enabled.

When vm_entry, the msr value of GLOBAL_CTRL on the hardware may be
different from trapped value "pmu->global_ctrl" written by the guest.

If the perf scheduler cross maps guest counter X to the host counter Y,
we have to enable the bit Y in GLOBAL_CTRL before vm_entry rather than X.

>
> This just cannot be right.