linux-kernel - Re: MSR access API uses in KVM x86

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Message-ID: <aAeqRk8fk8mvutw2@google.com>
Date: Tue, 22 Apr 2025 07:40:06 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Xin Li <xin@...or.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>
Subject: Re: MSR access API uses in KVM x86

On Mon, Apr 21, 2025, Xin Li wrote:
> It looks to me that MSR access API uses in KVM x86 are NOT consistent;
> sometimes {wr,rd}msrl() are used and sometimes native_{wr,rd}msrl() are
> used.
> 
> Was there a reason that how a generic or native MSR API was chosen?

I doubt anyone knows for sure; that'd likely require a time travelling device
and/or telepathic abilities :-)

> In my opinion KVM should use the native MSR APIs, which can streamline
> operations and potentially improve performance by avoiding the overhead
> associated with generic MSR API indirect calls when CONFIG_XEN_PV=y.

As Jürgen pointed out, they aren't indirect calls.  Though IIUC, there is still
a direct CALL and thus a RET when PARAVIRT_XXL=Y.

I agree that using PV APIs in KVM doesn't make much sense, as running KVM in a
XEN PV guest doesn't seem like something we should optimize for, if it's even
supported.  So if we end up churning all of the rdmsr/wrmsr macros, I have no
objection to switching to native variants.

Though if we do that, it would be nice if there's a way to avoid the "native_"
prefix everywhere, for the sake of readability.