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

[Eric Blake]

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 <address@hidden>
> ---
>  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 <address@hidden>

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

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