linux-kernel - Re: [PATCH v4 2/2] KVM: nSVM: implement ondemand allocation of the nested state

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Date:   Sun, 20 Sep 2020 09:16:02 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4 2/2] KVM: nSVM: implement ondemand allocation of the
 nested state

On Sat, Sep 19, 2020 at 05:09:09PM +0200, Paolo Bonzini wrote:
> On 17/09/20 18:29, Sean Christopherson wrote:
> >> +				vcpu->arch.efer = old_efer;
> >> +				kvm_make_request(KVM_REQ_OUT_OF_MEMORY, vcpu);
> > I really dislike KVM_REQ_OUT_OF_MEMORY.  It's redundant with -ENOMEM and
> > creates a huge discrepancy with respect to existing code, e.g. nVMX returns
> > -ENOMEM in a similar situation.
> 
> Maxim, your previous version was adding some error handling to
> kvm_x86_ops.set_efer.  I don't remember what was the issue; did you have
> any problems propagating all the errors up to KVM_SET_SREGS (easy),
> kvm_set_msr (harder) etc.?

I objected to letting .set_efer() return a fault.  A relatively minor issue is
the code in vmx_set_efer() that handles lack of EFER because technically KVM
can emulate EFER.SCE+SYSCALL without supporting EFER in hardware.  Returning
success/'0' would avoid that particular issue.  My primary concern is that I'd
prefer not to add another case where KVM can potentially ignore a fault
indicated by a helper, a la vmx_set_cr4().

To that end, I'd be ok with adding error handling to .set_efer() if KVM
enforces, via WARN in one of the .set_efer() call sites, that SVM/VMX can only
return negative error codes, i.e. let SVM handle the -ENOMEM case but disallow
fault injection.  It doesn't actually change anything, but it'd give me a warm
fuzzy feeling.