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

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Message-ID: <87k0wui2rs.fsf@vitty.brq.redhat.com>
Date:   Wed, 16 Sep 2020 09:44:55 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Wei Huang <wei.huang2@....com>,
        "Dr. David Alan Gilbert" <dgilbert@...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

Wei Huang <wei.huang2@....com> writes:

> 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.

Some CPUID functions have indicies (e.g. 0x00000004, 0x8000001d, ...)
and each existing index requires a separate entry. E.g. on my AMD EPYC
7401P host I can see:

# cpuid -r -1 | wc -l
53

Hyper-V emulation accounts for 11 leaves (we don't currently set them
all in QEMU but eventually we will). Also, we sometimes set both Intel
and AMD cpuid leaves in QEMU (David can probably elaborate on which/why)
and this all adds up. '80' seems to be an easy target to hit even with
existing CPUs :-)

-- 
Vitaly