linux-kernel - Re: [RFC PATCH 0/2] KVM: VMX: Fix VM entry failure on PT_MODE_HOST

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Date:   Mon, 29 Aug 2022 17:33:42 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Wang, Wei W" <wei.w.wang@...el.com>
Cc:     "Li, Xiaoyao" <xiaoyao.li@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "linux-perf-users@...r.kernel.org" <linux-perf-users@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [RFC PATCH 0/2] KVM: VMX: Fix VM entry failure on
 PT_MODE_HOST_GUEST while host is using PT

On Mon, Aug 29, 2022, Wang, Wei W wrote:
> On Thursday, August 25, 2022 4:56 PM, Xiaoyao Li wrote:
>  #if defined(CONFIG_PERF_EVENTS) && defined(CONFIG_CPU_SUP_AMD)
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index d7f8331d6f7e..195debc1bff1 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1125,37 +1125,29 @@ static inline void pt_save_msr(struct pt_ctx *ctx, u32 addr_range)
> 
>  static void pt_guest_enter(struct vcpu_vmx *vmx)
>  {
> -       if (vmx_pt_mode_is_system())
> +       struct perf_event *event;
> +
> +       if (vmx_pt_mode_is_system() ||
> +           !(vmx->pt_desc.guest.ctl & RTIT_CTL_TRACEEN))

I don't think the host should trace the guest in the host/guest mode just because
the guest isn't tracing itself.  I.e. the host still needs to turn off it's own
tracing.

>                 return;
> 
> -       /*
> -        * 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);
> -       if (vmx->pt_desc.guest.ctl & RTIT_CTL_TRACEEN) {
> -               wrmsrl(MSR_IA32_RTIT_CTL, 0);
> -               pt_save_msr(&vmx->pt_desc.host, vmx->pt_desc.num_address_ranges);
> -               pt_load_msr(&vmx->pt_desc.guest, vmx->pt_desc.num_address_ranges);
> -       }
> +       event = pt_get_curr_event();
> +       perf_event_disable(event);
> +       vmx->pt_desc.host_event = event;

This is effectively what I suggested[*], the main difference being that my version
adds dedicated enter/exit helpers so that perf can skip save/restore of the other
MSRs.  It's easy to extend if perf needs to hand back an event to complete the "exit.

	bool guest_trace_enabled = vmx->pt_desc.guest.ctl & RTIT_CTL_TRACEEN;
	
	vmx->pt_desc.host_event = intel_pt_guest_enter(guest_trace_enabled);


and then on exit

	bool guest_trace_enabled = vmx->pt_desc.guest.ctl & RTIT_CTL_TRACEEN;

	intel_pt_guest_exit(vmx->pt_desc.host_event, guest_trace_enabled);

[*] https://lore.kernel.org/all/YwecducnM%2FU6tqJT@google.com

> +       pt_load_msr(&vmx->pt_desc.guest, vmx->pt_desc.num_address_ranges);
> }