Re: [PATCH v2 4/6] RISC-V: Implement multi-letter ISA extension probing framework

["Heiko Stübner" <heiko@sntech.de>]

Am Dienstag, 15. Februar 2022, 10:12:53 CET schrieb Atish Kumar Patra:
> On Mon, Feb 14, 2022 at 3:22 PM Atish Kumar Patra <atishp@rivosinc.com> wrote:
> >
> > On Mon, Feb 14, 2022 at 2:22 PM Heiko Stübner <heiko@sntech.de> wrote:
> > >
> > > Am Montag, 14. Februar 2022, 21:42:32 CET schrieb Atish Patra:
> > > > On Mon, Feb 14, 2022 at 12:24 PM Heiko Stübner <heiko@sntech.de> wrote:
> > > > >
> > > > > Am Montag, 14. Februar 2022, 21:14:13 CET schrieb Atish Patra:
> > > > > > On Mon, Feb 14, 2022 at 12:06 PM Heiko Stübner <heiko@sntech.de> wrote:
> > > > > > >
> > > > > > > Am Donnerstag, 10. Februar 2022, 22:40:16 CET schrieb Atish Patra:
> > > > > > > > Multi-letter extensions can be probed using exising
> > > > > > > > riscv_isa_extension_available API now. It doesn't support versioning
> > > > > > > > right now as there is no use case for it.
> > > > > > > > Individual extension specific implementation will be added during
> > > > > > > > each extension support.
> > > > > > > >
> > > > > > > > Signed-off-by: Atish Patra <atishp@rivosinc.com>
> > > > > > >
> > > > > > > Tested-by: Heiko Stuebner <heiko@sntech.de>
> > > > > > >
> > > > > > >
> > > > > > > By the way, does a similar parsing exist for opensbi as well?
> > > > > > > Things like svpbmt as well as zicbom have CSR bits controlling how
> > > > > > > these functions should behave (enabling them, etc), so I guess
> > > > > > > opensbi also needs to parse the extensions from the ISA string?
> > > > > > >
> > > > > > >
> > > > > >
> > > > > > No. Currently, OpenSBI relies on the CSR read/write & trap method to
> > > > > > identify the extensions [1].
> > > > > >
> > > > > > https://github.com/riscv-software-src/opensbi/blob/master/lib/sbi/sbi_hart.c#L404
> > > > >
> > > > > I guess my question is more, who is supposed to set CBIE, CBCFE bits in the
> > > > > ENVCFG CSR. I.e. at it's default settings CMO instructions will cause
> > > > > illegal instructions until the level above does allow them.
> > > > >
> > > > > When the kernel wants to call a cache-invalidate, from my reading menvcfg
> > > > > needs to be modified accordingly - which would fall in SBI's court?
> > > > >
> > > >
> > > > I think so. I had the same question for the SSTC extension as well.
> > > > This is what I currently do:
> > > >
> > > > 1. Detect menvcfg first, detect stimecmp
> > > > 2. Enable SSTC feature only if both are available
> > > > 3. Set the STCE bit in menvcfg if SSTC is available
> > > >
> > > > Here is the patch
> > > > https://github.com/atishp04/opensbi/commit/e6b185821e8302bffdceb4633b413252e0de4889
> > >
> > > Hmm, the CBO fields are defined as WARL (write any, read legal),
> > > so I guess some sort of trap won't work here.
> > >
> >
> > Correct. Traps for extensions that introduce new CSRs.
> > I was suggesting setting the corresponding bits in MENVCFG and reading
> > it again to check if it sticks.
> >
> > > The priv-spec only points to the cmo-spec for these bits and the cmo-spec
> > > does not specifiy what the value should be when cmo is not present.
> > >
> > >
> > > > > > In the future, zicbom can be detected in the same manner. However,
> > > > > > svpbmt is a bit tricky as it doesn't
> > > > > > define any new CSR. Do you think OpenSBI needs to detect svpbmt for any reason ?
> > > > >
> > > > > There is the PBMTE bit in MENVCFG, which I found while looking through the
> > > > > zicbom-parts, which is supposed to "control wheter svpbmt is available for
> > > > > use". So I guess the question is the same as above :-)
> > > > >
> > > >
> > > > PBMTE bit in MENVCFG says if PBMTE bit is available or not. OpenSBI
> > > > needs other way to
> > > > detect if PBMTE is available.
> > > >
> > > > That's why, I think MENVCFG should be set correctly by the hardware
> > > > upon reset. What do you think
> > > > about that ? I couldn't find anything related to the reset state for menvcfg.
> > >
> > > me neither. Both the priv-spec as well as the cmobase spec do not
> > > specifiy any reset-values it seems.
> > >
> > I have raised an issue in the ISA spec.
> > https://github.com/riscv/riscv-isa-manual/issues/820
> >
> > > So I guess in the Qemu case, Qemu needs to set that bit when
> > > its svpbmt extension is enabled?
> > >
> >
> > We can do that if the priv spec is modified to allow that.
> >
> 
> As per Greg's response, hardware is not expected to do that.
> So we have to dynamically detect the extensions in OpenSBI and write to menvcfg.
> 
> I am not sure what needs to be done for CBIE bits as it both flush(01)
> or invalidate(11) are valid values

From looking at the security remark in the cmo-spec, I guess flush would be
the appropriate thing to do?

"Until a modified cache block has updated memory, a CBO.INVAL instruction may expose stale data values
in memory if the CSRs are programmed to perform an invalidate operation. This behavior may result in a
security hole if lower privileged level software performs an invalidate operation and accesses sensitive
information in memory."

But also do we actually _want_ to enable cmo always ... Greg was talking
about backwards compatiblity in his response as well.