Re: [PATCH v2 4/4] x86/cpuid: Advertise SERIALIZE by default to guests

[Jan Beulich <jbeulich@suse.com>]

On 15.12.2021 23:21, Andrew Cooper wrote:
> I've played with SERIALIZE, TSXLDTRK, MOVDIRI and MOVDIR64 on real hardware,
> and they all seem fine, including emulation support.
> 
> SERIALIZE exists specifically to have a userspace usable serialising operation
> without other side effects.  (The only other two choices are CPUID which is a
> VMExit under virt and clobbers 4 registers, and IRET-to-self which very slow
> and consumes content from the stack.)
> 
> TSXLDTRK is a niche TSX feature, and TSX itself is niche outside of demos of
> speculative sidechannels.  Leave the feature opt-in until a usecase is found,
> in an effort to preempt the multiple person years of effort it has taken to
> mop up TSX issues impacting every processor line.
> 
> MOVDIRI and MOVDIR64 are harder to judge.  They're architectural building
> blocks towards ENQCMD{,S} without obvious usecases on their own.  They're of
> no use to domains without PCI devices, so leave them opt-in for now.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

Acked-by: Jan Beulich <jbeulich@suse.com>