Re: [Xen-devel] Is: PVH dom0 - MWAIT detection logic to get deeper C-states exposed in ACPI AML code. Was:Re: [PATCH v2 10/30] xen/x86: Annotate VM applicability in featureset

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 02/17/2016 02:02 PM, Konrad Rzeszutek Wilk wrote:
> On Mon, Feb 15, 2016 at 03:41:41PM +0000, Andrew Cooper wrote:
> > On 15/02/16 15:02, Jan Beulich wrote:
> > > > > > On 15.02.16 at 15:53, <andrew.cooper3@xxxxxxxxxx> wrote:
> > > > On 15/02/16 14:50, Jan Beulich wrote:
> > > > > > > > On 15.02.16 at 15:38, <andrew.cooper3@xxxxxxxxxx> wrote:
> > > > > > On 15/02/16 09:20, Jan Beulich wrote:
> > > > > > > > > > On 12.02.16 at 18:42, <andrew.cooper3@xxxxxxxxxx> wrote:
> > > > > > > > On 12/02/16 17:05, Jan Beulich wrote:
> > > > > > > > > > > > On 05.02.16 at 14:42, <andrew.cooper3@xxxxxxxxxx> wrote:
> > > > > > > > > >   #define X86_FEATURE_MWAITX        ( 3*32+29) /*   MWAIT extension
> > > > > > > > (MONITORX/MWAITX) */
> > > > > > > > > Why not exposed to HVM (also for _MWAIT as I now notice)?
> > > > > > > > Because that is a good chunk of extra work to support.  We would need to
> > > > > > > > use 4K monitor widths, and extra p2m handling.
> > > > > > > I don't understand: The base (_MWAIT) feature being exposed to
> > > > > > > guests today, and kernels making use of the feature when available
> > > > > > > suggests to me that things work. Are you saying you know
> > > > > > > otherwise? (And if there really is a reason to mask the feature all of
> > > > > > > the sudden, this should again be justified in the commit message.)
> > > > > > PV guests had it clobbered by Xen in traps.c
> > > > > >
> > > > > > HVM guests have:
> > > > > >
> > > > > > vmx.c:
> > > > > >      case EXIT_REASON_MWAIT_INSTRUCTION:
> > > > > >      case EXIT_REASON_MONITOR_INSTRUCTION:
> > > > > > [...]
> > > > > >      hvm_inject_hw_exception(TRAP_invalid_op, HVM_DELIVER_NO_ERROR_CODE);
> > > > > >          break;
> > > > > >
> > > > > > and svm.c:
> > > > > >      case VMEXIT_MONITOR:
> > > > > >      case VMEXIT_MWAIT:
> > > > > >          hvm_inject_hw_exception(TRAP_invalid_op, HVM_DELIVER_NO_ERROR_CODE);
> > > > > >          break;
> > > > > >
> > > > > > I don't see how a guest could actually use this feature.
> > > > > Do you see the respective intercepts getting enabled anywhere?
> > > > > (I don't outside of nested code, which I didn't check in detail.)
> > > > Yes - the intercepts are always enabled to prevent the guest actually
> > > > putting the processor to sleep.
> > > Hmm, you're right, somehow I've managed to ignore the relevant
> > > lines grep reported. Yet - how do things work then, without the
> > > MWAIT feature flag currently getting cleared?

We whitelist CPUID0x00000001.ecx features in 
libxc/xc_cpuid_x86.c:xc_cpuid_hvm_policy() so MWAIT is never set.
-boris


> > I have never observed it being used.  Do you have some local patches in
> > the SLES hypervisor?
> >
> > There is some gross layer violation in xen/enlighten.c to pretend that
> > MWAIT is present to trick the ACPI code into evaluating _CST() methods
> > to report back to Xen.  (This is yet another PV-ism which will cause a
> > headache for a DMLite dom0)
> Yes indeed. CC-ing Roger, and Boris.
>
> > ~Andrew
> >
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxx
> > http://lists.xen.org/xen-devel

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