Re: [PATCH v4 10/38] perf/x86: Support switch_guest_ctx interface

[Peter Zijlstra]

On Fri, Apr 25, 2025 at 09:06:26AM -0400, Liang, Kan wrote:
> 
> 
> On 2025-04-25 7:15 a.m., Peter Zijlstra wrote:
> > On Mon, Mar 24, 2025 at 05:30:50PM +0000, Mingwei Zhang wrote:
> >> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> >>
> >> Implement switch_guest_ctx interface for x86 PMU, switch PMI to dedicated
> >> KVM_GUEST_PMI_VECTOR at perf guest enter, and switch PMI back to
> >> NMI at perf guest exit.
> >>
> >> Signed-off-by: Xiong Zhang <xiong.y.zhang@xxxxxxxxxxxxxxx>
> >> Signed-off-by: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> >> Tested-by: Yongwei Ma <yongwei.ma@xxxxxxxxx>
> >> Signed-off-by: Mingwei Zhang <mizhang@xxxxxxxxxx>
> >> ---
> >>  arch/x86/events/core.c | 12 ++++++++++++
> >>  1 file changed, 12 insertions(+)
> >>
> >> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> >> index 8f218ac0d445..28161d6ff26d 100644
> >> --- a/arch/x86/events/core.c
> >> +++ b/arch/x86/events/core.c
> >> @@ -2677,6 +2677,16 @@ static bool x86_pmu_filter(struct pmu *pmu, int cpu)
> >>  	return ret;
> >>  }
> >>  
> >> +static void x86_pmu_switch_guest_ctx(bool enter, void *data)
> >> +{
> >> +	u32 guest_lvtpc = *(u32 *)data;
> >> +
> >> +	if (enter)
> >> +		apic_write(APIC_LVTPC, guest_lvtpc);
> >> +	else
> >> +		apic_write(APIC_LVTPC, APIC_DM_NMI);
> >> +}
> > 
> > This, why can't it use x86_pmu.guest_lvtpc here and call it a day? Why
> > is that argument passed around through the generic code only to get back
> > here?
> 
> The vector has to be from the KVM. However, the current interfaces only
> support KVM read perf variables, e.g., perf_get_x86_pmu_capability and
> perf_get_hw_event_config.
> We need to add an new interface to allow the KVM write a perf variable,
> e.g., perf_set_guest_lvtpc.

But all that should remain in x86, there is no reason what so ever to
leak this into generic code.