Re: [Xen-devel] [PATCH v5 1/7] x86/hvm: allow ASID flush when v != current

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Fri, Feb 28, 2020 at 04:47:57PM +0100, Jan Beulich wrote:
> On 28.02.2020 16:27, Roger Pau Monné wrote:
> > On Fri, Feb 28, 2020 at 02:29:09PM +0100, Jan Beulich wrote:
> >> On 19.02.2020 18:43, Roger Pau Monne wrote:
> >>> Current implementation of hvm_asid_flush_vcpu is not safe to use
> >>> unless the target vCPU is either paused or the currently running one,
> >>> as it modifies the generation without any locking.
> >>
> >> Indeed, but the issue you're taking care of is highly theoretical:
> >> I don't think any sane compiler will split writes of the fields
> >> to multiple insns. It would be nice if this was made clear here.
> > 
> > What about adding:
> > 
> >>> Fix this by using atomic operations when accessing the generation
> >>> field, both in hvm_asid_flush_vcpu_asid and other ASID functions. This
> >>> allows to safely flush the current ASID generation. Note that for the
> >>> flush to take effect if the vCPU is currently running a vmexit is
> >>> required.
> > 
> > "Most compilers will already do such writes and reads as a single
> > instruction, so the usage of atomic operations is mostly used as a
> > safety measure."
> > 
> > Here?
> 
> Could you perhaps start with "Compilers will normally ..." I'm fine
> with the rest, it's just that "most compilers" still feels like
> an understatement.

Sure, that's fine.

Thanks, Roger.

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