Re: [PATCH v3 2/2] riscv: T-Head: Test availability bit before enabling MAE errata

[Conor Dooley]

On Mon, Apr 08, 2024 at 09:55:48AM +0200, Christoph Müllner wrote:
> On Mon, Apr 8, 2024 at 9:37 AM Yangyu Chen <cyy@xxxxxxxxxxxx> wrote:
> > > On Apr 8, 2024, at 14:00, Christoph Müllner <christoph.muellner@xxxxxxxx> wrote:
> > > On Mon, Apr 8, 2024 at 3:58 AM Yangyu Chen <cyy@xxxxxxxxxxxx> wrote:
> > >> On 2024/4/8 05:32, Christoph Müllner wrote:
> > >>> T-Head's memory attribute extension (XTheadMae) (non-compatible
> > >>> equivalent of RVI's Svpbmt) is currently assumed for all T-Head harts.
> > >>> However, QEMU recently decided to drop acceptance of guests that write
> > >>> reserved bits in PTEs.
> > >>> As XTheadMae uses reserved bits in PTEs and Linux applies the MAE errata
> > >>> for all T-Head harts, this broke the Linux startup on QEMU emulations
> > >>> of the C906 emulation.
> > >>>
> > >>> This patch attempts to address this issue by testing the MAE-enable bit
> > >>> in the th.sxstatus CSR. This CSR is available in HW and can be
> > >>> emulated in QEMU.
> > >>>
> > >>> This patch also makes the XTheadMae probing mechanism reliable, because
> > >>> a test for the right combination of mvendorid, marchid, and mimpid
> > >>> is not sufficient to enable MAE.
> > >>>
> > >>> Reviewed-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
> > >>> Signed-off-by: Christoph Müllner <christoph.muellner@xxxxxxxx>

> > >>> @@ -28,11 +31,14 @@ static bool errata_probe_mae(unsigned int stage,
> > >>>      if (arch_id != 0 || impid != 0)
> > >>>              return false;
> > >>>
> > >>
> > >> I would raise a little concern about keeping this "if" statement for
> > >> arch_id and imp_id after we have probed it in this CSR way. I would like to
> > >> remove it and move the CSR probe earlier than RISCV_ALTERNATIVES.
> > >>
> > >> I added CC to guoren for more opinions.
> > >>
> > >> Even T-Head C908 comes in 2023, which supports RVV 1.0 and also keeps MAEE.
> > >> But it also supports Svpbmt, and we can perform the switch by clearing the
> > >> MAEE bit in CSR_TH_MXSTATUS in M-Mode Software.
> > >>
> > >> Moreover, T-Head MAEE may not be removed in the future of T-Head CPUs. We
> > >> can see something from the T-Head C908 User Manual that adds a Security bit
> > >> to MAEE. So, it might used in their own TEE implementation and will not be
> > >> removed.
> > >>
> > >> However, C908 has arch_id and impid, which are not equal to zero. You can
> > >> see it from the C908 boot log [2] from my patch to support K230 [3]. So, if
> > >> we have probed MAEE using CSR, you confirmed that this bit will also be set
> > >> in the C906 core. We can only probe it this way and no longer use arch_id
> > >> and imp_id. And if the arch_id and imp_id probes get removed, we should
> > >> also move the csr probe earlier than riscv alternatives.
> > >
> > > We keep the probing via arch_id==0&&impid==0 because we had that
> > > already in the kernel and don't want to break existing functionality.
> > >
> > > From the discussions that we had about the v1 and v2 of this series,
> > > my impression is that we should use DT properties instead of probing
> > > arch_id and impid. So, if C908 support is needed, this should probably
> > > be introduced using DT properties. The logic would then be:
> > > * if arch_id == 0 and impid == 0 then decide based on th.sxstatus.MAEE
> > > * else test if "xtheadmae" is in the DT
> > >
> > >
> >
> > I know about it, and Conor also mentioned adding this property to DT a few
> > months ago. I agree with this at that time. But for now, you have found the
> > T-Head document description about this, and we can probe MAE using CSR even
> > on C906. I think only probing in CSR will be a better way to do that. It
> > can maintain compatibility with some early cores, such as C906. And will
> > also support some new cores with non-zero arch_id and impl_id but may have
> > MAE enabled, such as C908.
> >
> > For future concerns, T-Head said from their documentation that
> > "Availability: The th.sxstatus CSR is available on all systems whose
> > mvendorid CSR holds a value of 0x5B7." [1] and this extension is frozen and
> > stable. I think it's okay to have MAE errara for some cpus whose impl_id
> > and arch_id are non-zero. And T-Head may have used this for their TEE, so
> > it might never be removed.
> 
> I wrote that specification. And yes, T-Head reviewed that part.
> But there is no guarantee for future cores.

Yeah, that is what I assumed. Unless its a 100% certainty that this bit
will always have this meaning, we can't unconditionally assume that it
does.

> > Since it might never be removed, if some vendor uses it and makes it hard
> > to run the mainline kernel since it requires setting CSR in M-Mode software
> > or changing the DT, they may be hard to change for some security reasons
> > for TEE, I think it's not right

> The question is: why should the kernel have to care about that?
> This can all be addressed (hidden) in FW, where core-specific
> routines can test the required bits in vendor CSRs and set DT properties
> that match the core's configuration.

I'm also not super inclined to care about it requiring changes in
firmware, because the last time I checked T-Head's SDK (and therefore
the vendors') use a version of OpenSBI that cannot even run mainline and
needs to be updated to begin with.

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