linux-kernel - Re: [PATCH 3/6] x86/msr: minimize usage of native

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Message-ID: <aB54c5ajYkGZ1sPi@liuwe-devbox-ubuntu-v2.tail21d00.ts.net>
Date: Fri, 9 May 2025 21:49:39 +0000
From: Wei Liu <wei.liu@...nel.org>
To: Juergen Gross <jgross@...e.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org,
	linux-hyperv@...r.kernel.org, kvm@...r.kernel.org, xin@...or.com,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Vitaly Kuznetsov <vkuznets@...hat.com>,
	Sean Christopherson <seanjc@...gle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	xen-devel@...ts.xenproject.org
Subject: Re: [PATCH 3/6] x86/msr: minimize usage of native_*() msr access
 functions

On Tue, May 06, 2025 at 11:20:12AM +0200, Juergen Gross wrote:
> In order to prepare for some MSR access function reorg work, switch
> most users of native_{read|write}_msr[_safe]() to the more generic
> rdmsr*()/wrmsr*() variants.
> 
> For now this will have some intermediate performance impact with
> paravirtualization configured when running on bare metal, but this
> is a prereq change for the planned direct inlining of the rdmsr/wrmsr
> instructions with this configuration.
> 
> The main reason for this switch is the planned move of the MSR trace
> function invocation from the native_*() functions to the generic
> rdmsr*()/wrmsr*() variants. Without this switch the users of the
> native_*() functions would lose the related tracing entries.
> 
> Note that the Xen related MSR access functions will not be switched,
> as these will be handled after the move of the trace hooks.
> 
> Signed-off-by: Juergen Gross <jgross@...e.com>
> ---
>  arch/x86/hyperv/ivm.c      |  2 +-

Acked-by: Wei Liu <wei.liu@...nel.org>

> 
> diff --git a/arch/x86/hyperv/ivm.c b/arch/x86/hyperv/ivm.c
> index 09a165a3c41e..fe177a6be581 100644
> --- a/arch/x86/hyperv/ivm.c
> +++ b/arch/x86/hyperv/ivm.c
> @@ -319,7 +319,7 @@ int hv_snp_boot_ap(u32 cpu, unsigned long start_ip)
>  	asm volatile("movl %%ds, %%eax;" : "=a" (vmsa->ds.selector));
>  	hv_populate_vmcb_seg(vmsa->ds, vmsa->gdtr.base);
>  
> -	vmsa->efer = native_read_msr(MSR_EFER);
> +	rdmsrq(MSR_EFER, vmsa->efer);
>