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

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Date:   Tue, 15 Sep 2020 22:49:05 -0500
From:   Wei Huang <wei.huang2@....com>
To:     "Dr. David Alan Gilbert" <dgilbert@...hat.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>, 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

On 09/15 05:51, Dr. David Alan Gilbert wrote:
> * Vitaly Kuznetsov (vkuznets@...hat.com) wrote:
> > With QEMU and newer AMD CPUs (namely: Epyc 'Rome') the current limit for

Could you elaborate on this limit? On Rome, I counted ~35 CPUID functions which
include Fn0000_xxxx, Fn4000_xxxx and Fn8000_xxxx.

> > 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>
> 

I tested on two platforms and the patches worked fine. So no objection on the
design.

Tested-by: Wei Huang <wei.huang2@....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
>