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Re: [PATCH] x86/vpmu: fix race-condition in vpmu_load
- To: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 20 Sep 2022 16:54:19 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>, Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>
- Delivery-date: Tue, 20 Sep 2022 14:54:33 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 20.09.2022 16:26, Boris Ostrovsky wrote:
> On 9/20/22 4:01 AM, Jan Beulich wrote:
>> On 20.09.2022 00:42, Boris Ostrovsky wrote:
>>> It is saving vpmu data from current pcpu's MSRs for a remote vcpu so @v
>>> in vmx_find_msr() is not @current:
>>>
>>> vpmu_load()
>>> ...
>>> prev = per_cpu(last_vcpu, pcpu);
>>> vpmu_save_force(prev)
>>> core2_vpmu_save()
>>> __core2_vpmu_save()
>>> vmx_read_guest_msr()
>>> vmx_find_msr()
>>>
>>>
>>> The call to vmx_find_msr() was introduced by 755087eb9b10c. I wonder though
>>> whether
>>> this call is needed when code path above is executed (i.e. when we are
>>> saving
>>> remove vcpu)
>>
>> How could it not be needed? We need to obtain the guest value. The
>> thing I don't understand is why this forced saving is necessary,
>> when context_switch() unconditionally calls vpmu_switch_from().
>
>
> IIRC the logic is:
>
> 1. vcpuA runs on pcpu0
> 2. vcpuA is de-scheduled and is selected to run on pcpu1. It has not yet
> called vpmu_load() from pcpu1
The calling of vpmu_load() shouldn't matter here. What does matter is
that vpmu_save() was necessarily called already. Therefore I'm having
trouble seeing why ...
> 3. vcpuB is ready to run on pcpu0, calls vpmu_load()
> 4. vcpuB discovers that pcpu0's MSRs are still holding values from vcpuA
> 5. vcpuB calls vpmu_force_save(vcpuA) which stashes pcpu0's MSRs into vcpuA's
> vpmu context.
... forced saving would be necessary here. What's necessary at this
point is only the loading of vcpuB's MSR values.
Jan
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