linux-kernel - Re: [RFC PATCH 1/2] KVM: x86: Add a new system attribute for dynamic XSTATE component

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Date:   Thu, 25 Aug 2022 21:54:10 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>
Cc:     kvm@...r.kernel.org, pbonzini@...hat.com,
        linux-kernel@...r.kernel.org, yang.zhong@...el.com
Subject: Re: [RFC PATCH 1/2] KVM: x86: Add a new system attribute for dynamic
 XSTATE component

On Thu, Aug 25, 2022, Chang S. Bae wrote:
> On 8/25/2022 9:19 AM, Sean Christopherson wrote:
> > 
> > Adding new uAPI and new exports to eliminate one line of userspace code is not a
> > good tradeoff.  Am I missing something?  This really seems like solution looking
> > for a problem.
> 
> Well, then that's your call.
> 
> Yeah, that simplification is really minor. With this, I would rather think
> KVM wants enforcement before relaying the request to the host. That
> enforcement is unthinkable without supported_xcr0.

My point is that KVM needs supported_xcr0 (or similar behavior) regardless of
whether or not the kernel further restricts feature usage.

> But it looks like userspace is somehow trusted here for KVM.