linux-kernel - Re: [PATCH v3 8/8] KVM: nSVM: implement ondemand allocation of the nested state

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Message-ID: <58f9d2c1-0739-5b72-ee21-285474666c58@redhat.com>
Date:   Sat, 12 Sep 2020 18:32:37 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Jim Mattson <jmattson@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <joro@...tes.org>,
        Wanpeng Li <wanpengli@...cent.com>
Subject: Re: [PATCH v3 8/8] KVM: nSVM: implement ondemand allocation of the
 nested state

On 27/08/20 19:11, Maxim Levitsky wrote:
> +	hsave_page = alloc_page(GFP_KERNEL_ACCOUNT | __GFP_ZERO);
> +	if (!hsave_page)
> +		goto error;
> +

I think an error here should be just an internal error userspace exit,
or a -ENOMEM from KVM_RUN; not a #GP in the guest[1].  However, that's
difficult to plug into KVM.  Can you instead allocate nested state if
KVM_SET_CPUID2 sets the SVM bit?  Returning -ENOMEM from KVM_SET_CPUID2
is more likely to be something that userspace copes with.

I queued patches 1-5, and 7 for 5.9-rc.

Paolo

[1] Though in practice an order 0 allocation will never fail