From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
MIME-Version: 1.0
In-Reply-To: <CALMp9eQDWWAgmwnzZ0iTa2w+7-d_MjyOByOdfN0MjiHzaQWyEw@mail.gmail.com>
References: <20170208080917.24320-1-khuey@kylehuey.com> <20170208080917.24320-9-khuey@kylehuey.com>
 <CALCETrVS8ch-1harGv2NUy83ZLna8Vde1zNLxPTPaxuMexQMjw@mail.gmail.com>
 <CALMp9eSyJep6UPxeiZRWPrdzwvvcqJEbPPZ6C61EeHsvdyhMLg@mail.gmail.com>
 <6F48D384-B29C-41B4-83F1-B02FC2480205@amacapital.net> <CALMp9eQDWWAgmwnzZ0iTa2w+7-d_MjyOByOdfN0MjiHzaQWyEw@mail.gmail.com>
From: Andy Lutomirski <luto@kernel.org>
Date: Fri, 27 Jul 2018 14:05:40 -0700
Message-ID: <CALCETrVzH+9cJtxTHj300jDWx7S1bLxYCGX==7zHish9J5aFAQ@mail.gmail.com>
Subject: Re: [PATCH v14 8/9] KVM: x86: virtualize cpuid faulting
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
Sender: linux-kernel-owner@vger.kernel.org
To: Jim Mattson <jmattson@google.com>
Cc: Andy Lutomirski <luto@kernel.org>, Kyle Huey <me@kylehuey.com>, Robert O'Callahan <robert@ocallahan.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>, Paolo Bonzini <pbonzini@redhat.com>, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>, Jeff Dike <jdike@addtoit.com>, Richard Weinberger <richard@nod.at>, Alexander Viro <viro@zeniv.linux.org.uk>, Shuah Khan <shuah@kernel.org>, Dave Hansen <dave.hansen@linux.intel.com>, Borislav Petkov <bp@suse.de>, Peter Zijlstra <peterz@infradead.org>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, Len Brown <len.brown@intel.com>, "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>, Dmitry Safonov <dsafonov@virtuozzo.com>, David Matlack <dmatlack@google.com>, Nadav Amit <nadav.amit@gmail.com>, Andi Kleen <andi@firstfloor.org>, LKML <linux-kernel@vger.kernel.org>, user-mode-linux-devel@lists.sourceforge.net, "open list:USER-MODE LINUX (UML)" <user-mode-linux-user@lists.sourceforge.net>, Linux FS Devel <linux-fsdevel@vger.kernel.org>, "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@vger.kernel.org>, kvm list <kvm@vger.kernel.org>
List-ID: <user-mode-linux-devel.lists.sourceforge.net>

On Fri, Jul 27, 2018 at 2:03 PM, Jim Mattson <jmattson@google.com> wrote:
> On Fri, Jul 27, 2018 at 1:46 PM, Andy Lutomirski <luto@amacapital.net> wr=
ote:
>>> On Jul 27, 2018, at 1:28 PM, Jim Mattson <jmattson@google.com> wrote:
>>> Initializing this bit to zero helps with migration, but then if the
>>> CPUID faulting bits in both MSRs are set, userspace has to take pains
>>> to ensure that MSR_PLATFORM_INFO is restored first, or the
>>> MSR_MISC_FEATURES_ENABLES value will be rejected.
>>
>> The code could drop the constraint and just let the entry possibly fail =
if the MSRs are set wrong
>
> That would be an improvement, I think.

I personally don't know enough about the QEMU, kvmtool, etc
architecture to know whether this would be a good idea.

>
>>> I'm also concerned about the 0 in the "Maximum Non-Turbo Ratio" field
>>> feeding into someone's calculated TSC frequency.
>>
>> Hmm. I don=E2=80=99t have a good answer to that. Are there any real CPUs=
 that have this MSR but don=E2=80=99t have that field?
>
> No. The reason I bring this up is that a customer has code that
> expects to be able to derive the TSC frequency from this MSR (per
> Intel's instructions in SDM volume 3, section 18.7.3), and they were
> surprised to find that the MSR raises #GP on our platform. We're
> looking into cherry-picking this support from upstream as a start, but
> I know the customer would be unhappy to read zero from bits 15:8.

Does KVM *have* a concept of "maximum non-turbo frequency" of the
guest that it would make sense to expose here?  If so, presumably the
right solution is to expose it.