[Qemu-devel] [PATCH] target-i386: Don't cpu->migratable field when filtering features

[Qemu-devel] [PATCH] target-i386: Don't cpu->migratable field when filtering features

[Eduardo Habkost]

When explicitly enabling unmigratable flags using "-cpu host"
(e.g. "-cpu host,+invtsc"), the requested feature won't be
enabled because cpu->migratable is true by default.

This is inconsistent with all other CPU models, which don't have
the "migratable" option, making "+invtsc" work without the need
for extra options.

This happens because x86_cpu_filter_features() uses
cpu->migratable as argument for
x86_cpu_get_supported_feature_word(). This is not useful
because:
2) on "-cpu host" it only makes QEMU disable features that were
   explicitly enabled in the command-line;
1) on all the other CPU models, cpu->migratable is already false.

The fix is to just use 'false' as argument to
x86_cpu_get_supported_feature_word() in
x86_cpu_filter_features().

Note that:

* This won't change anything for people using using
  "-cpu host" or "-cpu host,migratable=<on|off>" (with no extra
  features) because the x86_cpu_get_supported_feature_word() call
  on the cpu->host_features check uses cpu->migratable as
  argument.
* This won't change anything for any CPU model except "host"
  because they all have cpu->migratable == false (and only "host"
  has the "migratable" property that allows it to be changed).
* This will only cange things for people using "-cpu host,+<feature>",
  where <feature> is a non-migratable feature. The only existing
  named migratable feature is "invtsc".

In other words, this change will only affect people using
"-cpu host,+invtsc" (that will now get what they asked for: the
invtsc flag will be enabled). All other use cases are unaffected.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 target-i386/cpu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index 754e575..d95514c 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -2248,7 +2248,7 @@ static int x86_cpu_filter_features(X86CPU *cpu)
 
     for (w = 0; w < FEATURE_WORDS; w++) {
         uint32_t host_feat =
-            x86_cpu_get_supported_feature_word(w, cpu->migratable);
+            x86_cpu_get_supported_feature_word(w, false);
         uint32_t requested_features = env->features[w];
         env->features[w] &= host_feat;
         cpu->filtered_features[w] = requested_features & ~env->features[w];
-- 
2.7.4

Re: [Qemu-devel] [PATCH] target-i386: Don't cpu->migratable field when filtering features

[Eric Blake]

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On 10/14/2016 02:28 PM, Eduardo Habkost wrote:

Subject line is missing a word; perhaps s/don't/don't read/

> When explicitly enabling unmigratable flags using "-cpu host"
> (e.g. "-cpu host,+invtsc"), the requested feature won't be
> enabled because cpu->migratable is true by default.
> 
> This is inconsistent with all other CPU models, which don't have
> the "migratable" option, making "+invtsc" work without the need
> for extra options.
> 
> This happens because x86_cpu_filter_features() uses
> cpu->migratable as argument for

s/as/as an/

> x86_cpu_get_supported_feature_word(). This is not useful
> because:
> 2) on "-cpu host" it only makes QEMU disable features that were
>    explicitly enabled in the command-line;
> 1) on all the other CPU models, cpu->migratable is already false.
> 
> The fix is to just use 'false' as argument to
> x86_cpu_get_supported_feature_word() in
> x86_cpu_filter_features().
> 
> Note that:
> 
> * This won't change anything for people using using
>   "-cpu host" or "-cpu host,migratable=<on|off>" (with no extra
>   features) because the x86_cpu_get_supported_feature_word() call
>   on the cpu->host_features check uses cpu->migratable as
>   argument.
> * This won't change anything for any CPU model except "host"
>   because they all have cpu->migratable == false (and only "host"
>   has the "migratable" property that allows it to be changed).
> * This will only cange things for people using "-cpu host,+<feature>",

s/cange/change/

>   where <feature> is a non-migratable feature. The only existing
>   named migratable feature is "invtsc".

s/migratable/non-migratable/ ?

> 
> In other words, this change will only affect people using
> "-cpu host,+invtsc" (that will now get what they asked for: the
> invtsc flag will be enabled). All other use cases are unaffected.
> 
> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> ---
>  target-i386/cpu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Love the commit:patch signal-to-noise ratio :)  But the lengthy
explanation is vital, so keep it that way.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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Re: [Qemu-devel] [PATCH] target-i386: Don't cpu->migratable field when filtering features

[Eduardo Habkost]

On Fri, Oct 14, 2016 at 02:36:52PM -0500, Eric Blake wrote:
> On 10/14/2016 02:28 PM, Eduardo Habkost wrote:
> 
> Subject line is missing a word; perhaps s/don't/don't read/

Changed to: "target-i386: Don't use cpu->migratable when
filtering features".

> 
> > When explicitly enabling unmigratable flags using "-cpu host"
> > (e.g. "-cpu host,+invtsc"), the requested feature won't be
> > enabled because cpu->migratable is true by default.
> > 
> > This is inconsistent with all other CPU models, which don't have
> > the "migratable" option, making "+invtsc" work without the need
> > for extra options.
> > 
> > This happens because x86_cpu_filter_features() uses
> > cpu->migratable as argument for
> 
> s/as/as an/

Fixed.

> 
> > x86_cpu_get_supported_feature_word(). This is not useful
> > because:
> > 2) on "-cpu host" it only makes QEMU disable features that were
> >    explicitly enabled in the command-line;
> > 1) on all the other CPU models, cpu->migratable is already false.
> > 
> > The fix is to just use 'false' as argument to
> > x86_cpu_get_supported_feature_word() in
> > x86_cpu_filter_features().

s/as/as an/ here, too.

> > 
> > Note that:
> > 
> > * This won't change anything for people using using
> >   "-cpu host" or "-cpu host,migratable=<on|off>" (with no extra
> >   features) because the x86_cpu_get_supported_feature_word() call
> >   on the cpu->host_features check uses cpu->migratable as
> >   argument.
> > * This won't change anything for any CPU model except "host"
> >   because they all have cpu->migratable == false (and only "host"
> >   has the "migratable" property that allows it to be changed).
> > * This will only cange things for people using "-cpu host,+<feature>",
> 
> s/cange/change/

Fixed.

> 
> >   where <feature> is a non-migratable feature. The only existing
> >   named migratable feature is "invtsc".
> 
> s/migratable/non-migratable/ ?

Fixed.

> 
> > 
> > In other words, this change will only affect people using
> > "-cpu host,+invtsc" (that will now get what they asked for: the
> > invtsc flag will be enabled). All other use cases are unaffected.
> > 
> > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

Ouch, the hurry to write the commit the message in time submit
this patch before the end of the day is noticeable. Thanks for
the review!

> > ---
> >  target-i386/cpu.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Love the commit:patch signal-to-noise ratio :)  But the lengthy
> explanation is vital, so keep it that way.

Yes, the rules and use cases behind the command-line options are
not trivial, so I wanted to be very explicit about the case the
patch affects, and the cases it doesn't affect.

> 
> Reviewed-by: Eric Blake <eblake@redhat.com>

Thanks!

-- 
Eduardo

Re: [Qemu-devel] [PATCH] target-i386: Don't cpu->migratable field when filtering features

[Ján Tomko]

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On Fri, Oct 14, 2016 at 04:28:14PM -0300, Eduardo Habkost wrote:
>When explicitly enabling unmigratable flags using "-cpu host"
>(e.g. "-cpu host,+invtsc"), the requested feature won't be
>enabled because cpu->migratable is true by default.
>

[...]

>
>Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
>---
> target-i386/cpu.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>

Tested-by: Ján Tomko <jtomko@redhat.com>

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