[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Message-ID: <ec95b28f-51a1-a9cf-7d72-a3a865797c7d@intel.com>
Date: Thu, 23 Jun 2022 16:55:41 -0700
From: "Chang S. Bae" <chang.seok.bae@...el.com>
To: Dave Hansen <dave.hansen@...el.com>, <len.brown@...el.com>,
<tony.luck@...el.com>, <rafael.j.wysocki@...el.com>,
<reinette.chatre@...el.com>, <dan.j.williams@...el.com>
CC: <corbet@....net>, <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] Documentation/x86: Explain guest XSTATE permission
control
On 6/16/2022 3:49 PM, Dave Hansen wrote:
> On 6/16/22 14:22, Chang S. Bae wrote:
>> +In addition, a couple of extended options are provided for a VCPU thread.
>> +The VCPU XSTATE permission is separately controlled.
>> +
>> +-ARCH_GET_XCOMP_GUEST_PERM
>> +
>> + arch_prctl(ARCH_GET_XCOMP_GUEST_PERM, &features);
>> +
>> + ARCH_GET_XCOMP_GUEST_PERM is a variant of ARCH_GET_XCOMP_PERM. So it
>> + provides the same semantics and functionality but for VCPU.
>
> This touches on the "what", but not the "why". Could you explain in
> here both why this is needed and why an app might want to use it?
[ while studying on this a bit further, found a few things here ]
They (ARCH_{REQ|GET}_XCOMP_GUEST_PERM) provide a userspace VMM to
request & check guest permission.
In general, KVM looks to have an API as a set of ioctls [1]. A guest VMM
uses KVM_GET_DEVICE_ATTR::KVM_X86_XCOMP_GUEST_SUPP to query the
available features [2][3]. ARCH_GET_XCOMP_SUPP is not usable here
because KVM wants to control those exposed features [4] (via
KVM_SUPPORTED_XCR0).
But oddly this mask does not appear to be actively referenced by those
two arch_prctl options. I can see this ioctl attribute is currently
disconnected from these arch_prctl options.
Also I failed to find the documentation about this
KVM_X86_XCOMP_GUEST_SUPP interface:
$ git grep KVM_X86_XCOMP_GUEST_SUPP ./Documentation/
$
I guess people will be confused with having these two options only. I
think documenting this has to come along with these missing pieces (and
potential fix). So I'm inclined to drop this one at the moment.
Thanks,
Chang
[1] https://kernel.org/doc/html/latest/virt/kvm/index.html
[2]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/kvm/lib/x86_64/processor.c#n641
[3]
https://github.com/qemu/qemu/blob/58b53669e87fed0d70903e05cd42079fbbdbc195/target/i386/kvm/kvm.c#L428
[4]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/x86/kvm/x86.c#n9008
Powered by blists - more mailing lists