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Subject: Re: [PATCH v4 2/2] KVM: nSVM: implement ondemand allocation of the
 nested state
To:     Sean Christopherson <sean.j.christopherson@intel.com>
Cc:     Maxim Levitsky <mlevitsk@redhat.com>, kvm@vger.kernel.org,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@alien8.de>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>
References: <20200917101048.739691-1-mlevitsk@redhat.com>
 <20200917101048.739691-3-mlevitsk@redhat.com>
 <20200917162942.GE13522@sjchrist-ice>
 <d9c0d190-c6ea-2e21-92ca-2a53efb86a1d@redhat.com>
 <20200920161602.GA17325@linux.intel.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <c35cbaca-2c34-cd93-b589-d4ab782fc754@redhat.com>
Date:   Sun, 20 Sep 2020 18:42:37 +0200
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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().