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

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Message-ID: <20200916192236.GA623906@weilap>
Date:   Wed, 16 Sep 2020 14:22:36 -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 09/16 09:33, Dr. David Alan Gilbert wrote:
> * Wei Huang (wei.huang2@....com) wrote:
> > 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.
> 
> On my 7302P the output of:
>     cpuid -1 -r | wc -l
> 
> is 61, there is one line of header in there.
> 
> However in a guest I see more; and I think that's because KVM  tends to
> list the CPUID entries for a lot of disabled Intel features, even on
> AMD, e.g. 0x11-0x1f which AMD doesn't have, are listed in a KVM guest.
> Then you add the KVM CPUIDs at 4...0 and 4....1.
>

It is indeed a mixing bag. Some are added even though AMD CPU doesn't define
them. BTW I also believe that cpuid command lists more CPUIDs than the real
value of cpuid->nent in kvm_vcpu_ioctl_set_cpuid(2).

Anyway I don't have objection to this patchset.

> IMHO we should be filtering those out for at least two reasons:
>   a) They're wrong
>   b) We're probably not keeping the set of visible CPUID fields the same
>     when we move between host kernels, and that can't be good for
> migration.
> 
> Still, those are separate problems.
> 
> Dave
> 
> > > > 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
> > > 
> > 
> -- 
> Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK
>