linux-kernel - Re: [PATCH RFC 0/2] KVM: x86: allow for more CPUID entries

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Date:   Tue, 15 Sep 2020 17:51:31 +0100
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>, Wei Huang <whuang2@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC 0/2] KVM: x86: allow for more CPUID entries

* Vitaly Kuznetsov (vkuznets@...hat.com) wrote:
> With QEMU and newer AMD CPUs (namely: Epyc 'Rome') the current limit for
> KVM_MAX_CPUID_ENTRIES(80) is reported to be hit. Last time it was raised
> from '40' in 2010. We can, of course, just bump it a little bit to fix
> the immediate issue but the report made me wonder why we need to pre-
> allocate vcpu->arch.cpuid_entries array instead of sizing it dynamically.
> This RFC is intended to feed my curiosity.
> 
> Very mildly tested with selftests/kvm-unit-tests and nothing seems to
> break. I also don't have access to the system where the original issue
> was reported but chances we're fixing it are very good IMO as just the
> second patch alone was reported to be sufficient.
> 
> Reported-by: Dr. David Alan Gilbert <dgilbert@...hat.com>

Oh nice, I was just going to bump the magic number :-)

Anyway, this seems to work for me, so:

Tested-by: Dr. David Alan Gilbert <dgilbert@...hat.com>

> Vitaly Kuznetsov (2):
>   KVM: x86: allocate vcpu->arch.cpuid_entries dynamically
>   KVM: x86: bump KVM_MAX_CPUID_ENTRIES
> 
>  arch/x86/include/asm/kvm_host.h |  4 +--
>  arch/x86/kvm/cpuid.c            | 55 ++++++++++++++++++++++++---------
>  arch/x86/kvm/x86.c              |  1 +
>  3 files changed, 43 insertions(+), 17 deletions(-)
> 
> -- 
> 2.25.4
> 
-- 
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK