linux-kernel - Re: [PATCH 3/3] KVM: x86: correct mwait and monitor emulation

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Date:	Wed, 18 Jun 2014 11:30:07 -0700
From:	Eric Northup <digitaleric@...gle.com>
To:	Nadav Amit <nadav.amit@...il.com>
Cc:	Nadav Amit <namit@...technion.ac.il>,
	Paolo Bonzini <pbonzini@...hat.com>, gleb@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	KVM <kvm@...r.kernel.org>, joro@...tes.org
Subject: Re: [PATCH 3/3] KVM: x86: correct mwait and monitor emulation

Quoting Gabriel's post http://www.spinics.net/lists/kvm/msg103792.html :

[...]

> E.g., OS X 10.5 *does* check CPUID, and panics if it doesn't find it.
> It needs the MONITOR cpuid flag to be on, *and* the actual
> instructions to work.




On Wed, Jun 18, 2014 at 11:23 AM, Nadav Amit <nadav.amit@...il.com> wrote:
> On 6/18/14, 8:59 PM, Eric Northup wrote:
>>
>> On Wed, Jun 18, 2014 at 7:19 AM, Nadav Amit <namit@...technion.ac.il>
>> wrote:
>>>
>>> mwait and monitor are currently handled as nop. Considering this
>>> behavior, they
>>> should still be handled correctly, i.e., check execution conditions and
>>> generate
>>> exceptions when required. mwait and monitor may also be executed in
>>> real-mode
>>> and are not handled in that case.  This patch performs the emulation of
>>> monitor-mwait according to Intel SDM (other than checking whether
>>> interrupt can
>>> be used as a break event).
>>>
>>> Signed-off-by: Nadav Amit <namit@...technion.ac.il>
>>> ---
>>>   arch/x86/kvm/emulate.c | 41 +++++++++++++++++++++++++++++++++++++++--
>>>   arch/x86/kvm/svm.c     | 22 ++--------------------
>>>   arch/x86/kvm/vmx.c     | 27 +++++++++++----------------
>>>   3 files changed, 52 insertions(+), 38 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
>>> index ef7a5a0..424b58d 100644
>>> --- a/arch/x86/kvm/emulate.c
>>> +++ b/arch/x86/kvm/emulate.c
>>> @@ -3344,6 +3344,43 @@ static int em_bswap(struct x86_emulate_ctxt *ctxt)
>>>          return X86EMUL_CONTINUE;
>>>   }
>>>
>>> +static int em_monitor(struct x86_emulate_ctxt *ctxt)
>>> +{
>>> +       int rc;
>>> +       struct segmented_address addr;
>>> +       u64 rcx = reg_read(ctxt, VCPU_REGS_RCX);
>>> +       u64 rax = reg_read(ctxt, VCPU_REGS_RAX);
>>> +       u8 byte;
>>
>>
>> I'd request:
>>
>> u32 ebx, ecx, edx, eax = 1;
>> ctxt->opt->get_cpuid(ctxt, &eax, &ebx, &ecx, &edx);
>> if (!(ecx & FFL(MWAIT)))
>>          return emulate_ud(ctxt);
>>
>> and also in em_mwait.
>>
>
> I had similar implementation on previous version, which also checked on
> mwait whether "interrupt as break event" matches ECX value. However, I was
> under the impression that it was decided that MWAIT will always be emulated
> as NOP to avoid misbehaving VMs that ignore CPUID (see the discussion at
> http://www.spinics.net/lists/kvm/msg102766.html ).
>
> Nadav
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