On 14.06.2023 20:12, Andrew Cooper wrote: > On 13/06/2023 10:59 am, Jan Beulich wrote: >> On 12.06.2023 18:13, Andrew Cooper wrote: >>> The RSBA bit, "RSB Alternative", means that the RSB may use alternative >>> predictors when empty. From a practical point of view, this mean "Retpoline >>> not safe". >>> >>> Enhanced IBRS (officially IBRS_ALL in Intel's docs, previously IBRS_ATT) is >>> a >>> statement that IBRS is implemented in hardware (as opposed to the form >>> retrofitted to existing CPUs in microcode). >>> >>> The RRSBA bit, "Restricted-RSBA", is a combination of RSBA, and the eIBRS >>> property that predictions are tagged with the mode in which they were >>> learnt. >>> Therefore, it means "when eIBRS is active, the RSB may fall back to >>> alternative predictors but restricted to the current prediction mode". As >>> such, it's stronger statement than RSBA, but still means "Retpoline not >>> safe". >>> >>> CPUs are not expected to enumerate both RSBA and RRSBA. >>> >>> Add feature dependencies for EIBRS and RRSBA. While technically they're not >>> linked, absolutely nothing good can come of letting the guest see RRSBA >>> without EIBRS. Nor a guest seeing EIBRS without IBRSB. Furthermore, we use >>> this dependency to simplify the max derivation logic. >>> >>> The max policies gets RSBA and RRSBA unconditionally set (with the EIBRS >>> dependency maybe hiding RRSBA). We can run any VM, even if it has been told >>> "somewhere you might run, Retpoline isn't safe". >>> >>> The default policies are more complicated. A guest shouldn't see both bits, >>> but it needs to see one if the current host suffers from any form of RSBA, >>> and >>> which bit it needs to see depends on whether eIBRS is visible or not. >>> Therefore, the calculation must be performed after sanitise_featureset(). >>> >>> Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com> >>> --- >>> CC: Jan Beulich <jbeul...@suse.com> >>> CC: Roger Pau Monné <roger....@citrix.com> >>> CC: Wei Liu <w...@xen.org> >>> >>> v3: >>> * Minor commit message adjustment. >>> * Drop changes to recalculate_cpuid_policy(). Deferred to a later series. >> With this dropped, with the title not saying "max/default", and with >> the description also not mentioning "live" policies at all, I don't >> think this patch is self-consistent (meaning in particular: leaving >> aside the fact that there's no way right now to requests e.g. both >> RSBA and RRSBA for a guest; aiui it is possible for Dom0). >> >> As you may imagine I'm also curious why you decided to drop this. > > Because when I tried doing levelling in Xapi, I remembered why I did it > the way I did in v1, and why the v2 way was wrong. > > Xen cannot safely edit what the toolstack provides, so must not.
And this is the part I don't understand: Why can't we correct the (EIBRS,RSBA,RRSBA) tuple to a combination that is "legal"? At least as long as ... > Instead, failing the set_policy() call is an option, and is what we want > to do longterm, ... we aren't there. > but also happens to be wrong too in this case. An admin > may know that a VM isn't using retpoline, and may need to migrate it > anyway for a number of reasons, so any safety checks need to be in the > toolstack, and need to be overrideable with something like --force. Possibly leading to an inconsistent policy exposed to a guest? I guess this may be the only option when we can't really resolve an ambiguity, but that isn't the case here, is it? > I don't really associate "derive policies" with anything other than the > system policies. Domain construction isn't any kind of derivation - > it's simply doing what the toolstack asks. Hmm, I see. To me, since we do certain adjustments, "derive" still fits there as well. But I'm not going to insist on a subject adjustment then, given that imo both ways of looking at things make some sense. Jan
