Re: MSR access API uses in KVM x86

[Xin Li <xin@zytor.com>]

On 4/22/2025 7:40 AM, 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

Right, I didn't notice such an optimization went in.

> a direct CALL and thus a RET when PARAVIRT_XXL=Y.

Correct.

> 
> 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.

Thanks for the confirmation.

> 
> 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.
> 

Yeah, I will think about better naming them.