On 11.06.2024 18:21, Roger Pau Monné wrote: > On Tue, Jun 11, 2024 at 04:53:22PM +0200, Jan Beulich wrote: >> On 11.06.2024 15:52, Roger Pau Monné wrote: >>> On Tue, Jun 11, 2024 at 01:52:58PM +0200, Jan Beulich wrote: >>>> On 11.06.2024 13:08, Roger Pau Monné wrote: >>>>> I really wonder whether Xen has enough information to figure out >>>>> whether a hole (MMIO region) is supposed to be accessed as UC or >>>>> something else. >>>> >>>> It certainly hasn't, and hence is erring on the (safe) side of forcing >>>> UC. >>> >>> Except that for the vesa framebuffer at least this is a bad choice :). >> >> Well, yes, that's where we want WC to be permitted. But for that we only >> need to avoid setting iPAT; we still can uniformly hand back UC. Except >> (as mentioned elsewhere earlier) if the guest uses MTRRs rather than PAT >> to arrange for WC. > > If we want to get this into 4.19, we likely want to go your proposed > approach then, as it's less risky. > > I think a comment would be helpful to note that the fix here to not > enforce iPAT by still return UC is mostly done to accommodate vesa > regions mapped with PAT attributes to use WC. > > I would also like to add some kind of note that special casing > !mfn_valid() might not be needed, but that removing it must be done > carefully to not cause regressions.
Hmm, in the meantime I have myself sufficiently convinced that with a small (hopefully easy / uncontroversial) change to ept_set_entry() I can arrange for having the guarantees that neither INVALID_MFN nor a truncated for of it can make it into the function, allowing the check to be dropped (as you had initially asked for). >>>> One caveat here that I forgot to >>>> mention before: MFNs taken out of EPT entries will never be INVALID_MFN, >>>> for >>>> the truncation that happens when populating entries. In that case we rely >>>> on >>>> mfn_valid() to be "rejecting" them. >>> >>> The only caller where mfns from EPT entries are passed to >>> epte_get_entry_emt() is in resolve_misconfig() AFAICT, and in that >>> case the EPT entry must be present for epte_get_entry_emt() to be >>> called. So it seems to me that epte_get_entry_emt() can never be >>> called from an mfn constructed from an INVALID_MFN EPT entry (but it's >>> worth adding an assert for it). >> >> Are you sure? I agree for the first of those two calls, but the second >> isn't quite as obvious. There we'd need to first prove that we will >> never create non-present super-page entries. Yet if I'm not mistaken >> for PoD we may create such. > > I should go look then, didn't know PoD would do that. I've meanwhile checked, and indeed we do. That's what with said prereq change I hope to make no longer be the case. Jan
