[Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

[Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

[Bandan Das]

By default, CPUID_EXT_MONITOR is enabled for some cpu models 
such as Opteron_G3. Disable it if kvm_enabled() is true since 
monitor/mwait aren't supported by KVM yet. 

Signed-off-by: Bandan Das <bsd@redhat.com>
---
There is no user visible side-effect to this behavior, the aim 
is to clean up the default flags that are not supported (yet).

 target-i386/cpu.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index 1a501d9..c83ba1c 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -1749,6 +1749,7 @@ static void cpu_x86_register(X86CPU *cpu, const char *name, Error **errp)
 
     if (kvm_enabled()) {
         def->features[FEAT_KVM] |= kvm_default_features;
+        def->features[FEAT_1_ECX] &= ~CPUID_EXT_MONITOR;
     }
     def->features[FEAT_1_ECX] |= CPUID_EXT_HYPERVISOR;
 
-- 
1.8.1.4

Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

[Bandan Das]

Forwarding message by Eduardo. I had misspelled nongnu.org in my first attempt!
The spaces/tab comment by Eduardo has been fixed. 

Eduardo Habkost <ehabkost@redhat.com> writes:
> 
> By default, CPUID_EXT_MONITOR is enabled for some cpu models 
> such as Opteron_G3. Disable it if kvm_enabled() is true since 
> monitor/mwait aren't supported by KVM yet. 
> 
> Signed-off-by: Bandan Das <bsd@redhat.com>

Interesting, I haven't noticed that TCG supports CPUID_EXT_MONITOR.

I believe that's yet another reason to make the KVM CPU models separate
classes from the TCG CPU models: because
"-machine ...,accel=kvm -cpu Foo" and "-machine ...,accel=tcg -cpu Foo"
_already_ have different meanings today and result in different CPUs.
Making them classes would just make the fact that they _are_ different
CPU models explicit.


> ---
> There is no user visible side-effect to this behavior, the aim 
> is to clean up the default flags that are not supported (yet).

There is one user-visible effect: "-cpu ...,enforce" will stop failing
because of missing KVM support for CPUID_EXT_MONITOR. But that's exactly
the point: there's no point in having CPU model definitions that would
never work as-is with neither TCG or KVM. This patch is changing the
meaning of (e.g.) "-machine ...,accel=kvm -cpu Opteron_G3" to match what
was already happening in practice.

> 
>  target-i386/cpu.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> index 1a501d9..c83ba1c 100644
> --- a/target-i386/cpu.c
> +++ b/target-i386/cpu.c
> @@ -1749,6 +1749,7 @@ static void cpu_x86_register(X86CPU *cpu, const char *name, Error **errp)
>  
>      if (kvm_enabled()) {
>          def->features[FEAT_KVM] |= kvm_default_features;
> +	def->features[FEAT_1_ECX] &= ~CPUID_EXT_MONITOR;

You are mixing spaces and tabs, here.

>      }
>      def->features[FEAT_1_ECX] |= CPUID_EXT_HYPERVISOR;
>  
> -- 
> 1.8.1.4
> 

-- 
Eduardo

> By default, CPUID_EXT_MONITOR is enabled for some cpu models 
> such as Opteron_G3. Disable it if kvm_enabled() is true since 
> monitor/mwait aren't supported by KVM yet. 
>
> Signed-off-by: Bandan Das <bsd@redhat.com>
> ---
> There is no user visible side-effect to this behavior, the aim 
> is to clean up the default flags that are not supported (yet).
>
>  target-i386/cpu.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> index 1a501d9..c83ba1c 100644
> --- a/target-i386/cpu.c
> +++ b/target-i386/cpu.c
> @@ -1749,6 +1749,7 @@ static void cpu_x86_register(X86CPU *cpu, const char *name, Error **errp)
>  
>      if (kvm_enabled()) {
>          def->features[FEAT_KVM] |= kvm_default_features;
> +        def->features[FEAT_1_ECX] &= ~CPUID_EXT_MONITOR;
>      }
>      def->features[FEAT_1_ECX] |= CPUID_EXT_HYPERVISOR;

Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

[Paolo Bonzini]

Il 25/05/2013 03:21, Bandan Das ha scritto:
> There is one user-visible effect: "-cpu ...,enforce" will stop failing
> because of missing KVM support for CPUID_EXT_MONITOR. But that's exactly
> the point: there's no point in having CPU model definitions that would
> never work as-is with neither TCG or KVM. This patch is changing the
> meaning of (e.g.) "-machine ...,accel=kvm -cpu Opteron_G3" to match what
> was already happening in practice.

But then -cpu Opteron_G3 does not match a "real" Opteron G3.  Is it
worth it?

Paolo

Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

[Eduardo Habkost]

On Sat, May 25, 2013 at 08:25:49AM +0200, Paolo Bonzini wrote:
> Il 25/05/2013 03:21, Bandan Das ha scritto:
> > There is one user-visible effect: "-cpu ...,enforce" will stop failing
> > because of missing KVM support for CPUID_EXT_MONITOR. But that's exactly
> > the point: there's no point in having CPU model definitions that would
> > never work as-is with neither TCG or KVM. This patch is changing the
> > meaning of (e.g.) "-machine ...,accel=kvm -cpu Opteron_G3" to match what
> > was already happening in practice.
> 
> But then -cpu Opteron_G3 does not match a "real" Opteron G3.  Is it
> worth it?

No models match a "real" CPU this way, because neither TCG or KVM
support all features supported by a real CPU. I ask the opposite
question: is it worth maintaining an "accurate" CPU model definition
that would never work without feature-bit tweaking in the command-line?

-- 
Eduardo

Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

[Paolo Bonzini]

Il 27/05/2013 14:09, Eduardo Habkost ha scritto:
> On Sat, May 25, 2013 at 08:25:49AM +0200, Paolo Bonzini wrote:
>> Il 25/05/2013 03:21, Bandan Das ha scritto:
>>> There is one user-visible effect: "-cpu ...,enforce" will stop failing
>>> because of missing KVM support for CPUID_EXT_MONITOR. But that's exactly
>>> the point: there's no point in having CPU model definitions that would
>>> never work as-is with neither TCG or KVM. This patch is changing the
>>> meaning of (e.g.) "-machine ...,accel=kvm -cpu Opteron_G3" to match what
>>> was already happening in practice.
>>
>> But then -cpu Opteron_G3 does not match a "real" Opteron G3.  Is it
>> worth it?
> 
> No models match a "real" CPU this way, because neither TCG or KVM
> support all features supported by a real CPU. I ask the opposite
> question: is it worth maintaining an "accurate" CPU model definition
> that would never work without feature-bit tweaking in the command-line?

It would work with TCG.  Changing TCG to KVM should not change hardware
if you use "-cpu ...,enforce", so it is right that it fails when
starting with KVM.

Paolo

Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

[Eduardo Habkost]

On Mon, May 27, 2013 at 02:21:36PM +0200, Paolo Bonzini wrote:
> Il 27/05/2013 14:09, Eduardo Habkost ha scritto:
> > On Sat, May 25, 2013 at 08:25:49AM +0200, Paolo Bonzini wrote:
> >> Il 25/05/2013 03:21, Bandan Das ha scritto:
> >>> There is one user-visible effect: "-cpu ...,enforce" will stop failing
> >>> because of missing KVM support for CPUID_EXT_MONITOR. But that's exactly
> >>> the point: there's no point in having CPU model definitions that would
> >>> never work as-is with neither TCG or KVM. This patch is changing the
> >>> meaning of (e.g.) "-machine ...,accel=kvm -cpu Opteron_G3" to match what
> >>> was already happening in practice.
> >>
> >> But then -cpu Opteron_G3 does not match a "real" Opteron G3.  Is it
> >> worth it?
> > 
> > No models match a "real" CPU this way, because neither TCG or KVM
> > support all features supported by a real CPU. I ask the opposite
> > question: is it worth maintaining an "accurate" CPU model definition
> > that would never work without feature-bit tweaking in the command-line?
> 
> It would work with TCG.  Changing TCG to KVM should not change hardware
> if you use "-cpu ...,enforce", so it is right that it fails when
> starting with KVM.
> 

Changing between KVM and TCG _does_ change hardware, today (with or
without check/enforce). All CPU models on TCG have features not
supported by TCG automatically removed. See the "if (!kvm_enabled())"
block at x86_cpu_realizefn().

(That's why I argue that we need separate classes/names for TCG and KVM
modes. Otherwise our predefined models get less useful as they will
require low-level feature-bit fiddling on the libvirt side to make them
work as expected.)

-- 
Eduardo

Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

[Paolo Bonzini]

Il 27/05/2013 15:07, Eduardo Habkost ha scritto:
>> Changing TCG to KVM should not change hardware
>> > if you use "-cpu ...,enforce", so it is right that it fails when
>> > starting with KVM.
>> > 
> Changing between KVM and TCG _does_ change hardware, today (with or
> without check/enforce). All CPU models on TCG have features not
> supported by TCG automatically removed. See the "if (!kvm_enabled())"
> block at x86_cpu_realizefn().

Perhaps (for "-cpu ...,enforce" or check) that's the real bug we have to
fix?

Paolo

> (That's why I argue that we need separate classes/names for TCG and KVM
> modes. Otherwise our predefined models get less useful as they will
> require low-level feature-bit fiddling on the libvirt side to make them
> work as expected.)
> 

Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

[Eduardo Habkost]

On Mon, May 27, 2013 at 03:14:25PM +0200, Paolo Bonzini wrote:
> Il 27/05/2013 15:07, Eduardo Habkost ha scritto:
> >> Changing TCG to KVM should not change hardware
> >> > if you use "-cpu ...,enforce", so it is right that it fails when
> >> > starting with KVM.
> >> > 
> > Changing between KVM and TCG _does_ change hardware, today (with or
> > without check/enforce). All CPU models on TCG have features not
> > supported by TCG automatically removed. See the "if (!kvm_enabled())"
> > block at x86_cpu_realizefn().
> 
> Perhaps (for "-cpu ...,enforce" or check) that's the real bug we have to
> fix?

It would be 100% accurate, but would it be useful? We will then have a
Opteron_G3 CPU model that can't be used reliably without low-level bit
fiddling, on either TCG or KVM mode.

Note that I am not completely against it. I agree that having a single
CPU model namespace with 100%-equivalent definitions for both TCG and
KVM modes would simplify the logic a lot. I am just not sure if it is
worth it.

(To be honest, I am more worried about the amount of time I will waste
trying to change the behavior of the TCG code. I am pretty sure I will
hear something like "why are you wasting your time trying to make
check/enforce work for TCG? Please leave TCG alone!".)

-- 
Eduardo

Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

[Bandan Das]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Mon, May 27, 2013 at 02:21:36PM +0200, Paolo Bonzini wrote:
>> Il 27/05/2013 14:09, Eduardo Habkost ha scritto:
>> > On Sat, May 25, 2013 at 08:25:49AM +0200, Paolo Bonzini wrote:
>> >> Il 25/05/2013 03:21, Bandan Das ha scritto:
>> >>> There is one user-visible effect: "-cpu ...,enforce" will stop failing
>> >>> because of missing KVM support for CPUID_EXT_MONITOR. But that's exactly
>> >>> the point: there's no point in having CPU model definitions that would
>> >>> never work as-is with neither TCG or KVM. This patch is changing the
>> >>> meaning of (e.g.) "-machine ...,accel=kvm -cpu Opteron_G3" to match what
>> >>> was already happening in practice.
>> >>
>> >> But then -cpu Opteron_G3 does not match a "real" Opteron G3.  Is it
>> >> worth it?
>> > 
>> > No models match a "real" CPU this way, because neither TCG or KVM
>> > support all features supported by a real CPU. I ask the opposite
>> > question: is it worth maintaining an "accurate" CPU model definition
>> > that would never work without feature-bit tweaking in the command-line?
>> 
>> It would work with TCG.  Changing TCG to KVM should not change hardware
>> if you use "-cpu ...,enforce", so it is right that it fails when
>> starting with KVM.
>> 
>
> Changing between KVM and TCG _does_ change hardware, today (with or
> without check/enforce). All CPU models on TCG have features not
> supported by TCG automatically removed. See the "if (!kvm_enabled())"
> block at x86_cpu_realizefn().

Yes, this is exactly why I was inclined to remove the monitor flag. 
We already have uses of kvm_enabled() to set (or remove) kvm specific stuff,
and this change is no different. I can see Paolo's point though, having 
a common definition probably makes sense too.


> (That's why I argue that we need separate classes/names for TCG and KVM
> modes. Otherwise our predefined models get less useful as they will
> require low-level feature-bit fiddling on the libvirt side to make them
> work as expected.)

Agreed. From a user's perspective, I think the more a CPU model "just works",
whether it's KVM or TCG, the better.

Bandan

Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

[Paolo Bonzini]

Il 28/05/2013 18:34, Bandan Das ha scritto:
> Eduardo Habkost <ehabkost@redhat.com> writes:
> 
>> On Mon, May 27, 2013 at 02:21:36PM +0200, Paolo Bonzini wrote:
>>> Il 27/05/2013 14:09, Eduardo Habkost ha scritto:
>>>> On Sat, May 25, 2013 at 08:25:49AM +0200, Paolo Bonzini wrote:
>>>>> Il 25/05/2013 03:21, Bandan Das ha scritto:
>>>>>> There is one user-visible effect: "-cpu ...,enforce" will stop failing
>>>>>> because of missing KVM support for CPUID_EXT_MONITOR. But that's exactly
>>>>>> the point: there's no point in having CPU model definitions that would
>>>>>> never work as-is with neither TCG or KVM. This patch is changing the
>>>>>> meaning of (e.g.) "-machine ...,accel=kvm -cpu Opteron_G3" to match what
>>>>>> was already happening in practice.
>>>>>
>>>>> But then -cpu Opteron_G3 does not match a "real" Opteron G3.  Is it
>>>>> worth it?
>>>>
>>>> No models match a "real" CPU this way, because neither TCG or KVM
>>>> support all features supported by a real CPU. I ask the opposite
>>>> question: is it worth maintaining an "accurate" CPU model definition
>>>> that would never work without feature-bit tweaking in the command-line?
>>>
>>> It would work with TCG.  Changing TCG to KVM should not change hardware
>>> if you use "-cpu ...,enforce", so it is right that it fails when
>>> starting with KVM.
>>>
>>
>> Changing between KVM and TCG _does_ change hardware, today (with or
>> without check/enforce). All CPU models on TCG have features not
>> supported by TCG automatically removed. See the "if (!kvm_enabled())"
>> block at x86_cpu_realizefn().
> 
> Yes, this is exactly why I was inclined to remove the monitor flag. 
> We already have uses of kvm_enabled() to set (or remove) kvm specific stuff,
> and this change is no different.

Do any of these affect something that is part of x86_def_t?

> I can see Paolo's point though, having 
> a common definition probably makes sense too.

>> (That's why I argue that we need separate classes/names for TCG and KVM
>> modes. Otherwise our predefined models get less useful as they will
>> require low-level feature-bit fiddling on the libvirt side to make them
>> work as expected.)
> 
> Agreed. From a user's perspective, I think the more a CPU model "just works",
> whether it's KVM or TCG, the better.

Yes, that's right.  But I think extending the same expectation to "-cpu
...,enforce" is not necessary, and perhaps even wrong for "-cpu
...,check" since it's only a warning rather than a fatal error.

Paolo

Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

[Andreas Färber]

Am 28.05.2013 18:46, schrieb Paolo Bonzini:
> Il 28/05/2013 18:34, Bandan Das ha scritto:
>> Eduardo Habkost <ehabkost@redhat.com> writes:
>>
>>> On Mon, May 27, 2013 at 02:21:36PM +0200, Paolo Bonzini wrote:
>>>> Il 27/05/2013 14:09, Eduardo Habkost ha scritto:
>>>>> On Sat, May 25, 2013 at 08:25:49AM +0200, Paolo Bonzini wrote:
>>>>>> Il 25/05/2013 03:21, Bandan Das ha scritto:
>>>>>>> There is one user-visible effect: "-cpu ...,enforce" will stop failing
>>>>>>> because of missing KVM support for CPUID_EXT_MONITOR. But that's exactly
>>>>>>> the point: there's no point in having CPU model definitions that would
>>>>>>> never work as-is with neither TCG or KVM. This patch is changing the
>>>>>>> meaning of (e.g.) "-machine ...,accel=kvm -cpu Opteron_G3" to match what
>>>>>>> was already happening in practice.
>>>>>>
>>>>>> But then -cpu Opteron_G3 does not match a "real" Opteron G3.  Is it
>>>>>> worth it?
>>>>>
>>>>> No models match a "real" CPU this way, because neither TCG or KVM
>>>>> support all features supported by a real CPU. I ask the opposite
>>>>> question: is it worth maintaining an "accurate" CPU model definition
>>>>> that would never work without feature-bit tweaking in the command-line?
>>>>
>>>> It would work with TCG.  Changing TCG to KVM should not change hardware
>>>> if you use "-cpu ...,enforce", so it is right that it fails when
>>>> starting with KVM.
>>>>
>>>
>>> Changing between KVM and TCG _does_ change hardware, today (with or
>>> without check/enforce). All CPU models on TCG have features not
>>> supported by TCG automatically removed. See the "if (!kvm_enabled())"
>>> block at x86_cpu_realizefn().
>>
>> Yes, this is exactly why I was inclined to remove the monitor flag. 
>> We already have uses of kvm_enabled() to set (or remove) kvm specific stuff,
>> and this change is no different.
> 
> Do any of these affect something that is part of x86_def_t?

The vendor comes to mind.

Andreas

>> I can see Paolo's point though, having 
>> a common definition probably makes sense too.
> 
>>> (That's why I argue that we need separate classes/names for TCG and KVM
>>> modes. Otherwise our predefined models get less useful as they will
>>> require low-level feature-bit fiddling on the libvirt side to make them
>>> work as expected.)
>>
>> Agreed. From a user's perspective, I think the more a CPU model "just works",
>> whether it's KVM or TCG, the better.
> 
> Yes, that's right.  But I think extending the same expectation to "-cpu
> ...,enforce" is not necessary, and perhaps even wrong for "-cpu
> ...,check" since it's only a warning rather than a fatal error.
> 
> Paolo
> 


-- 
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany
GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer; HRB 16746 AG Nürnberg

Re: [Qemu-devel] [PATCH] target-i386: Disable CPUID_EXT_MONITOR when KVM is enabled

[Eduardo Habkost]

(CCing libvirt people)

On Tue, May 28, 2013 at 06:48:52PM +0200, Andreas Färber wrote:
> Am 28.05.2013 18:46, schrieb Paolo Bonzini:
> > Il 28/05/2013 18:34, Bandan Das ha scritto:
> >> Eduardo Habkost <ehabkost@redhat.com> writes:
> >>
> >>> On Mon, May 27, 2013 at 02:21:36PM +0200, Paolo Bonzini wrote:
> >>>> Il 27/05/2013 14:09, Eduardo Habkost ha scritto:
> >>>>> On Sat, May 25, 2013 at 08:25:49AM +0200, Paolo Bonzini wrote:
> >>>>>> Il 25/05/2013 03:21, Bandan Das ha scritto:
> >>>>>>> There is one user-visible effect: "-cpu ...,enforce" will stop failing
> >>>>>>> because of missing KVM support for CPUID_EXT_MONITOR. But that's exactly
> >>>>>>> the point: there's no point in having CPU model definitions that would
> >>>>>>> never work as-is with neither TCG or KVM. This patch is changing the
> >>>>>>> meaning of (e.g.) "-machine ...,accel=kvm -cpu Opteron_G3" to match what
> >>>>>>> was already happening in practice.
> >>>>>>
> >>>>>> But then -cpu Opteron_G3 does not match a "real" Opteron G3.  Is it
> >>>>>> worth it?
> >>>>>
> >>>>> No models match a "real" CPU this way, because neither TCG or KVM
> >>>>> support all features supported by a real CPU. I ask the opposite
> >>>>> question: is it worth maintaining an "accurate" CPU model definition
> >>>>> that would never work without feature-bit tweaking in the command-line?
> >>>>
> >>>> It would work with TCG.  Changing TCG to KVM should not change hardware
> >>>> if you use "-cpu ...,enforce", so it is right that it fails when
> >>>> starting with KVM.
> >>>>
> >>>
> >>> Changing between KVM and TCG _does_ change hardware, today (with or
> >>> without check/enforce). All CPU models on TCG have features not
> >>> supported by TCG automatically removed. See the "if (!kvm_enabled())"
> >>> block at x86_cpu_realizefn().
> >>
> >> Yes, this is exactly why I was inclined to remove the monitor flag. 
> >> We already have uses of kvm_enabled() to set (or remove) kvm specific stuff,
> >> and this change is no different.
> > 
> > Do any of these affect something that is part of x86_def_t?
> 
> The vendor comes to mind.

I believe we can still consider the "vendor" field a special one: if
other components care about the TCG/KVM difference regarding the
"vendor" field, they can simply set "vendor" explicitly on the
command-line.

> >> I can see Paolo's point though, having 
> >> a common definition probably makes sense too.
> > 

Paolo is convincing me that keeping the rest of the features exactly the
same on TCG and KVM modes (and making check/enforce work for TCG as
well) would simplify the logic a lot. This will add a little extra work
for libvirt, that will probably need to use "-cpu Opteron_G3,-monitor"
once it implements enforce-mode (to make sure the results really match
existing libvirt assumptions about the Opteron_G* models), but it is
probably worth it.

I will give it a try and send a proposal soon.


> >>> (That's why I argue that we need separate classes/names for TCG and KVM
> >>> modes. Otherwise our predefined models get less useful as they will
> >>> require low-level feature-bit fiddling on the libvirt side to make them
> >>> work as expected.)
> >>
> >> Agreed. From a user's perspective, I think the more a CPU model "just works",
> >> whether it's KVM or TCG, the better.
> > 
> > Yes, that's right.  But I think extending the same expectation to "-cpu
> > ...,enforce" is not necessary, and perhaps even wrong for "-cpu
> > ...,check" since it's only a warning rather than a fatal error.
> > 
> > Paolo
> > 
> 
> 
> -- 
> SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg, Germany
> GF: Jeff Hawn, Jennifer Guild, Felix Imendörffer; HRB 16746 AG Nürnberg

-- 
Eduardo