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

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Message-ID: <c35cbaca-2c34-cd93-b589-d4ab782fc754@redhat.com>
Date:   Sun, 20 Sep 2020 18:42:37 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.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 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().