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 02/18/2016 10:12 AM, Andrew Cooper wrote:
> On 18/02/16 15:02, Roger Pau Monné wrote:
> > El 17/2/16 a les 20:02, Konrad Rzeszutek Wilk ha escrit:
> > > 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?
> > > > 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.
> > Yes, all this is indeed not very nice, and we would ideally like to get
> > rid of it on PVHv2.
We will have to come up with something else: AIUI the whole point of 
xen_check_mwait() is to come up with proper ECX and EDX values for the 
MWAIT CPUID leaf. Those value are expected to be reported from 
xen_cpuid() pv_op so that acpi_processor_ffh_state_probe_cpu() can set C 
states structures properly.
The problem is that PVH executes CPUID instruction natively. (And so 
this must have been broken on PVHv1 as well).
-boris


> > Could we use the acpica tools (acpidump/acpixtract/acpiexec/...) in
> > order to fetch this information from user-space and send it to Xen using
> > privcmd?
> >
> > AFAIK those tools work on most OSes (or at least the ones we care about
> > as Dom0).
> In general, we can't rely on userspace evaluation of AML.
>
> For trivial AML which evaluates to a constant, it could be interpreted
> by userspace, but anything accessing system resources will need
> evaluating by the kernel.
>
> ~Andrew

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
http://lists.xen.org/xen-devel