On 24/01/18 13:34, Woodhouse, David wrote: > On Wed, 2018-01-24 at 13:12 +0000, Andrew Cooper wrote: >> + * Squash the domid and vcpu id together for comparason > *comparison > >> + * efficiency. We could in principle stash and compare the >> struct >> + * vcpu pointer, but this risks a false alias if a domain has >> died >> + * and the same 4k page gets reused for a new vcpu. >> + */ > Isn't that also true if the domain has died and its domain-id gets re- > used?
In principle, yes. However, a toolstack needs to have non-default behaviour[1] to reuse a domid without wrapping around 32k. > >> + unsigned int next_id = (((unsigned int)nextd->domain_id << 16) | >> + (uint16_t)next->vcpu_id); > I am loath to suggest *more* tweakables, but given the IBPB cost is > there any merit in having a mode which does it only if the *domain* is > different, regardless of vcpu_id? This would only be a win if you were regularly cross-scheduling vcpus from the same domain, which case you've probably other issues to be worried about. > If a given domain is running on HT siblings, it ought to be doing its > own mitigation — setting STIBP for userspace if it wants, ensuring its > own kernel is safe by having IBRS set or using retpoline, etc. ~Andrew [1] Is this trying to be a subtle hint? _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
