Messages in this thread | | | Date | Thu, 19 Oct 2023 08:15:03 +0100 | From | Marc Zyngier <> | Subject | Re: [PATCH] arm64: Independently update HDFGRTR_EL2 and HDFGWTR_EL2 |
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On Thu, 19 Oct 2023 04:36:15 +0100, Anshuman Khandual <anshuman.khandual@arm.com> wrote: > > > > On 10/18/23 18:10, Marc Zyngier wrote: > > On Wed, 18 Oct 2023 04:00:07 +0100, > > Anshuman Khandual <anshuman.khandual@arm.com> wrote: > >> > >> Currently PMSNEVFR_EL1 system register read, and write access EL2 traps are > >> disabled, via setting the same bit (i.e 62) in HDFGRTR_EL2, and HDFGWTR_EL2 > >> respectively. Although very similar, bit fields are not exact same in these > >> two EL2 trap configure registers particularly when it comes to read-only or > >> write-only accesses such as ready-only 'HDFGRTR_EL2.nBRBIDR' which needs to > >> be set while enabling BRBE on NVHE platforms. Using the exact same bit mask > >> fields for both these trap register risk writing into their RESERVED areas, > >> which is undesirable. > > > > Sorry, I don't understand at all what you are describing. You seem to > > imply that the read and write effects of the FGT doesn't apply the > > same way. But my reading of the ARM ARM is that behave completely > > symmetrically. > > > > Also, what is nBRBIDR doing here? It is still set to 0. What > > 'RESERVED' state are you talking about? > > Let's observe the following example which includes the nBRBIDR problem, > mentioned earlier. > > Read access trap configure > > HDFGRTR_EL2[59] - nBRBIDR > HDFGRTR_EL2[58] - PMCEIDn_EL0 > > Write access trap configure > > HDFGWTR_EL2[59:58] - RES0 > > Because BRBIDR_EL1 and PMCEID<N>_EL0 are read only registers they don't > have corresponding entries in HDFGWTR_EL2 for write trap configuration. > > Using the exact same value contained in 'x0' both for HDFGRTR_EL2, and > HDFGWTR_EL2 will be problematic in case it contains bit fields that are > available only in one of the registers but not in the other. > > If 'x0' contains nBRBIDR being set, it will be okay for HDFGRTR_EL2 but > might not be okay for HDFGWTR_EL2 where it will get into RESERVED areas.
None of which matters for this patch. You keep arguing about something that does not exist in the change you're proposing.
[...]
> I should have given more details in the commit message but hope > you have some context now, but please do let me know if there > is something still missing.
What is missing is a useful patch. This one just obfuscates things for no particular purpose. If you have a useful change to contribute, please send that instead (your BRBE change). We don't need an extra, standalone and pointless patch such as this one.
Thanks,
M.
-- Without deviation from the norm, progress is not possible.
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