Re: [Qemu-devel] [PATCH] target-i386: enable x2apic by default on more recent CPU models

["Andreas Färber" <afaerber@suse.de>]

Am 21.01.2014 11:03, schrieb Paolo Bonzini:
> Il 20/01/2014 23:13, Andreas Färber ha scritto:
>> I don't like the argument that we can put arbitrary stuff in our model
>> definitions and rely on TCG not having implemented it to make it
>> correct. Is x2apic something that TCG can never implement for some
>> reason? Then that needs a better explanation. Otherwise, is there no
>> criteria we can add this flag for when kvm_enabled()?
> 
> We already do that for other bits (e.g. XSAVE/OSXSAVE),

Please point me to the commit, a search for xsave did not come up with a
commit changing such a thing - either it did not go through my queue or
it slipped me through: Bugs are no excuse to produce more bugs.

> and in fact it
> is the same that we do for KVM: the KVM_GET_SUPPORTED_CPUID result is
> used to trim the generic feature bits.

Our model definitions are the place to put stuff that real CPUs have.
Either the CPU has it or it doesn't. If it does, then this patch is
fully correct and it's TCG's job to mask things out. If we're adding
artificial flags to the generic model definitions just to make KVM
faster, then it is wrong - we have a choice of post_initialize and
realize hooks for that.

Andreas

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