Re: [PATCH 4/5] SVM: sync VM-exit perf counters with known VM-exit reasons

From: Andrew Cooper <amc96@srcf.net>
To: Jan Beulich <jbeulich@suse.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Cc: "Andrew Cooper" <andrew.cooper3@citrix.com>,
	"George Dunlap" <george.dunlap@citrix.com>,
	"Wei Liu" <wl@xen.org>, "Roger Pau Monné" <roger.pau@citrix.com>
Subject: Re: [PATCH 4/5] SVM: sync VM-exit perf counters with known VM-exit reasons
Date: Fri, 17 Dec 2021 15:02:57 +0000	[thread overview]
Message-ID: <60deae51-6ccd-dd90-869a-db6099e21002@srcf.net> (raw)
In-Reply-To: <c94bc336-fdee-43af-540e-06e0904d8db7@suse.com>

On 03/12/2021 12:06, Jan Beulich wrote:
> This has gone out of sync over time, resulting in NPF and XSETBV exits
> incrementing the same counter. Introduce a simplistic mechanism to
> hopefully keep things in better sync going forward.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> ---
> Given their large (and growing) number, I wonder whether we shouldn't
> fold "SVMexits" and "vmexits". They can't both be active at the same
> time.

Oh yeah - that's just silly having them split like this, especially as
there's no associated element name.

> --- a/xen/include/asm-x86/perfc_defn.h
> +++ b/xen/include/asm-x86/perfc_defn.h
> @@ -11,8 +11,8 @@ PERFCOUNTER_ARRAY(exceptions,
>  PERFCOUNTER_ARRAY(vmexits,              "vmexits", VMX_PERF_EXIT_REASON_SIZE)
>  PERFCOUNTER_ARRAY(cause_vector,         "cause vector", VMX_PERF_VECTOR_SIZE)
>  
> -#define VMEXIT_NPF_PERFC 141
> -#define SVM_PERF_EXIT_REASON_SIZE (1+141)
> +#define VMEXIT_NPF_PERFC 143
> +#define SVM_PERF_EXIT_REASON_SIZE (VMEXIT_NPF_PERFC + 1)

How does this work in the first place?  perfc_incra() is still passed 1024.

Furthermore, it's already worse than this.

401/402 are AVIC exits, and 403 is an SEV exit.  We've also gained -2 as
"busy" for transient SEV events too.

~Andrew