From: Avi Kivity <avi@redhat.com>
To: Stefan Fritsch <sf@sfritsch.de>
Cc: kvm@vger.kernel.org
Subject: Re: How to do fast accesses to LAPIC TPR under kvm?
Date: Mon, 22 Oct 2012 16:10:30 +0200 [thread overview]
Message-ID: <508553D6.8030709@redhat.com> (raw)
In-Reply-To: <201210200039.16412.sf@sfritsch.de>
On 10/20/2012 12:39 AM, Stefan Fritsch wrote:
> On Thursday 18 October 2012, Avi Kivity wrote:
>> On 10/18/2012 11:35 AM, Gleb Natapov wrote:
>> > You misunderstood the description. V_INTR_MASKING=1 means that
>> > CR8 writes are not propagated to real HW APIC.
>> >
>> > But KVM does not trap access to CR8 unconditionally. It enables
>> > CR8 intercept only when there is pending interrupt in IRR that
>> > cannot be immediately delivered due to current TPR value. This
>> > should eliminate 99% of CR8 intercepts.
>>
>> Right. You will need to expose the alternate encoding of cr8 (IIRC
>> lock mov reg, cr0) on AMD via cpuid, but otherwise it should just
>> work. Be aware that this will break cross-vendor migration.
>
> I get an exception and I am not sure why:
>
> kvm_entry: vcpu 0
> kvm_exit: reason write_cr8 rip 0xd0203788 info 0 0
> kvm_emulate_insn: 0:d0203788: f0 0f 22 c0 (prot32)
> kvm_inj_exception: #UD (0x0)
>
> This is qemu-kvm 1.1.2 on Linux 3.2.
>
> When I look at arch/x86/kvm/emulate.c (both the current and the v3.2
> version), I don't see any special case handling for "lock mov reg,
> cr0" to mean "mov reg, cr8".
emulate.c will #UD is the Lock flag is missing in the instruction decode
table.
> Before I spend lots of time on debugging my code, can you verify if
> the alternate encoding of cr8 is actually supported in kvm or if it is
> maybe missing? Thanks in advance.
With the decode table fix I think it should work.
--
error compiling committee.c: too many arguments to function
next prev parent reply other threads:[~2012-10-22 14:10 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-17 19:24 How to do fast accesses to LAPIC TPR under kvm? Stefan Fritsch
2012-10-18 6:13 ` Jan Kiszka
2012-10-18 7:43 ` Stefan Fritsch
2012-10-18 9:35 ` Gleb Natapov
2012-10-18 12:27 ` Avi Kivity
2012-10-18 17:25 ` Stefan Fritsch
2012-10-19 22:39 ` Stefan Fritsch
2012-10-22 14:10 ` Avi Kivity [this message]
2012-10-24 9:19 ` Stefan Fritsch
2012-10-25 13:34 ` Avi Kivity
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