Re: [RFC PATCH 2/2] KVM: VMX: Stop/resume host PT before/after VM entry when PT_MODE_HOST_GUEST

Xiaoyao Li
2022-08-26T06:32:21Z

On 8/25/2022 11:59 PM, Sean Christopherson wrote:
> On Thu, Aug 25, 2022, Xiaoyao Li wrote:
>> On 8/25/2022 11:34 PM, Sean Christopherson wrote:
>>> On Thu, Aug 25, 2022, Xiaoyao Li wrote:
>>>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>>>> index d7f8331d6f7e..3e9ce8f600d2 100644
>>>> --- a/arch/x86/kvm/vmx/vmx.c
>>>> +++ b/arch/x86/kvm/vmx/vmx.c
>>>> @@ -38,6 +38,7 @@
>>>> #include <asm/fpu/api.h>
>>>> #include <asm/fpu/xstate.h>
>>>> #include <asm/idtentry.h>
>>>> +#include <asm/intel_pt.h>
>>>> #include <asm/io.h>
>>>> #include <asm/irq_remapping.h>
>>>> #include <asm/kexec.h>
>>>> @@ -1128,13 +1129,19 @@ static void pt_guest_enter(struct vcpu_vmx *vmx)
>>>> if (vmx_pt_mode_is_system())
>>>> return;
>>>> + /*
>>>> + * Stop Intel PT on host to avoid vm-entry failure since
>>>> + * VM_ENTRY_LOAD_IA32_RTIT_CTL is set
>>>> + */
>>>> + intel_pt_stop();
>>>> +
>>>> /*
>>>> * GUEST_IA32_RTIT_CTL is already set in the VMCS.
>>>> * Save host state before VM entry.
>>>> */
>>>> rdmsrl(MSR_IA32_RTIT_CTL, vmx->pt_desc.host.ctl);
>>>
>>> KVM's manual save/restore of MSR_IA32_RTIT_CTL should be dropped.
>>
>> No. It cannot. Please see below.
>>
>>> If PT/RTIT can
>>> trace post-VMXON, then intel_pt_stop() will disable tracing and intel_pt_resume()
>>> will restore the host's desired value.
>>
>> intel_pt_stop() and intel_pt_resume() touches host's RTIT_CTL only when host
>> enables/uses Intel PT. Otherwise, they're just noop. In this case, we cannot
>> assume host's RTIT_CTL is zero (only the RTIT_CTL.TraceEn is 0). After
>> VM-exit, RTIT_CTL is cleared, we need to restore it.
>
> But ensuring the RTIT_CTL.TraceEn=0 is all that's needed to make VM-Entry happy,
> and if the host isn't using Intel PT, what do we care if other bits that, for all
> intents and purposes are ignored, are lost across VM-Entry/VM-Exit? I gotta
> imaging the perf will fully initialize RTIT_CTL if it starts using PT.

Personally, I agree with it.

But I'm not sure if there is a criteria that host context needs to be
unchanged after being virtualized.

> Actually, if the host isn't actively using Intel PT, can KVM avoid saving the
> other RTIT MSRs?

I don't think it's a good idea that it requires PT driver never and
won't rely on the previous value of PT MSRs. But it's OK if handing it
over to perf as the idea you gave below.

> Even better, can we hand that off to perf? I really dislike KVM making assumptions
> about perf's internal behavior. E.g. can this be made to look like

you mean let perf subsystem to do the context save/restore staff of host
and KVM focuses on save/restore of guest context, right?

I would like to see comment from perf folks on this and maybe need their
help on how to implement.

> intel_pt_guest_enter(vmx->pt_desc.guest.ctl & RTIT_CTL_TRACEEN);
>
> and
>
> intel_pt_guest_exit(vmx->pt_desc.guest.ctl & RTIT_CTL_TRACEEN);
>