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

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Message-ID: <801d663a-d448-401d-a957-c9825aca06c3@suse.com>
Date: Tue, 22 Apr 2025 16:59:21 +0200
From: Juergen Gross <jgross@...e.com>
To: Sean Christopherson <seanjc@...gle.com>, 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 22.04.25 16:40, Sean Christopherson wrote:
> 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.

Shudder. :-D

No, definitely not supported.


Juergen

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