linux-kernel - Re: [PATCH V10 01/18] perf/core: Use static_call to optimize perf_guest_info

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Message-ID: <YSkfeXcd6nUT3AOJ@google.com>
Date:   Fri, 27 Aug 2021 17:23:05 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Zhu Lingshan <lingshan.zhu@...el.com>
Cc:     peterz@...radead.org, pbonzini@...hat.com, bp@...en8.de,
        vkuznets@...hat.com, wanpengli@...cent.com, jmattson@...gle.com,
        joro@...tes.org, kan.liang@...ux.intel.com, ak@...ux.intel.com,
        wei.w.wang@...el.com, eranian@...gle.com, liuxiangdong5@...wei.com,
        linux-kernel@...r.kernel.org, x86@...nel.org, kvm@...r.kernel.org,
        like.xu.linux@...il.com, boris.ostrvsky@...cle.com,
        Like Xu <like.xu@...ux.intel.com>,
        Will Deacon <will@...nel.org>, Marc Zyngier <maz@...nel.org>,
        Guo Ren <guoren@...nel.org>, Nick Hu <nickhu@...estech.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-csky@...r.kernel.org, linux-riscv@...ts.infradead.org,
        xen-devel@...ts.xenproject.org
Subject: Re: [PATCH V10 01/18] perf/core: Use static_call to optimize
 perf_guest_info_callbacks

On Fri, Aug 06, 2021, Zhu Lingshan wrote:
> @@ -2944,18 +2966,21 @@ static unsigned long code_segment_base(struct pt_regs *regs)
>  
>  unsigned long perf_instruction_pointer(struct pt_regs *regs)
>  {
> -	if (perf_guest_cbs && perf_guest_cbs->is_in_guest())
> -		return perf_guest_cbs->get_guest_ip();
> +	unsigned long ip = static_call(x86_guest_get_ip)();
> +
> +	if (likely(!ip))

Pivoting on ip==0 isn't correct, it's perfectly legal for a guest to execute
from %rip=0.  Unless there's some static_call() magic that supports this with a
default function:
	
	if (unlikely(!static_call(x86_guest_get_ip)(&ip)))
		regs->ip + code_segment_base(regs)

	return ip;

The easiest thing is keep the existing:

	if (unlikely(static_call(x86_guest_state)()))
		return static_call(x86_guest_get_ip)();

	return regs->ip + code_segment_base(regs);

It's an extra call for PMIs in guest, but I don't think any of the KVM folks care
_that_ much about the performance in this case.

> +		ip = regs->ip + code_segment_base(regs);
>  
> -	return regs->ip + code_segment_base(regs);
> +	return ip;
>  }