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

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Date:   Mon, 21 Sep 2020 10:53:17 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     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 Sun, 2020-09-20 at 18:42 +0200, Paolo Bonzini wrote:
> On 20/09/20 18:16, Sean Christopherson wrote:
> > > 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.
> 
> So did I, and that's why we get KVM_REQ_OUT_OF_MEMORY.  But it was more
> of an "it's ugly and it ought not to fail" thing than something I could
> pinpoint.
> 
> It looks like we agree, but still we have to choose the lesser evil?
> 
> Paolo
> 
> > 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().

The thing is that kvm_emulate_wrmsr injects #GP when kvm_set_msr returns any non zero value,
and returns 1 which means keep on going if I understand correctly (0 is userspace exit,
negative value would be a return to userspace with an error)

So the question is if we have other wrmsr handlers which return negative value, and would
be affected by changing kvm_emulate_wrmsr to pass through the error value.
I am checking the code now.

I do agree now that this is the *correct* solution to this problem.

Best regards,
	Maxim Levitsky