Re: [PATCH v4 7/9] riscv: vector: adjust minimum Vector requirement to ZVE32X

[Conor Dooley]

On Thu, Apr 18, 2024 at 08:52:56AM -0700, Eric Biggers wrote:
> Hi Conor,
> 
> On Thu, Apr 18, 2024 at 12:02:10PM +0100, Conor Dooley wrote:
> > +CC Eric, Jerry
> > 
> > On Fri, Apr 12, 2024 at 02:49:03PM +0800, Andy Chiu wrote:
> > > Make has_vector take one argument. This argument represents the minimum
> > > Vector subextension that the following Vector actions assume.
> > > 
> > > Also, change riscv_v_first_use_handler(), and boot code that calls
> > > riscv_v_setup_vsize() to accept the minimum Vector sub-extension,
> > > ZVE32X.
> > > 
> > > Most kernel/user interfaces requires minimum of ZVE32X. Thus, programs
> > > compiled and run with ZVE32X should be supported by the kernel on most
> > > aspects. This includes context-switch, signal, ptrace, prctl, and
> > > hwprobe.
> > > 
> > > One exception is that ELF_HWCAP returns 'V' only if full V is supported
> > > on the platform. This means that the system without a full V must not
> > > rely on ELF_HWCAP to tell whether it is allowable to execute Vector
> > > without first invoking a prctl() check.
> > > 
> > > Signed-off-by: Andy Chiu <andy.chiu@xxxxxxxxxx>
> > > Acked-by: Joel Granados <j.granados@xxxxxxxxxxx>
> > 
> > I'm not sure that I like this patch to be honest. As far as I can tell,
> > every user here of has_vector(ext) is ZVE32X, so why bother actually
> > having an argument?
> > 
> > Could we just document that has_vector() is just a tyre kick of "is
> > there a vector unit and are we allowed to use it", and anything
> > requiring more than the bare-minimum (so zve32x?)must explicitly check
> > for that form of vector using riscv_has_extension_[un]likely()?
> > 
> > Finally, the in-kernel crypto stuff or other things that use
> > can_use_simd() to check for vector support - do they all function correctly
> > with all of the vector flavours? I don't understand the vector
> > extensions well enough to evaluate that - I know that they do check for
> > the individual extensions like Zvkb during probe but don't have anything
> > for the vector version (at least in the chacha20 and sha256 glue code).
> > If they don't, then we need to make sure those drivers do not probe with
> > the cut-down variants.
> 
> As far as I know, none of the RISC-V vector crypto code has been tested with
> Zve* yet.  Currently it always checks for VLEN >= 128, which should exclude most
> Zve* implementations.
> 
> Currently it doesn't check for EEW >= 64, even though it sometimes assumes that.
> It looks like a check for EEW >= 64 needs to be added in order to exclude Zve32x
> and Zve32f implementations that don't support EEW == 64.

Cool, glad I asked then :)

> If it would be useful to do so, we should be able to enable some of the code
> with a smaller VLEN and/or EEW once it has been tested in those configurations.
> Some of it should work, but some of it won't be able to work.  (For example, the
> SHA512 instructions require EEW==64.)
> 
> Also note that currently all the RISC-V vector crypto code only supports riscv64
> (XLEN=64).  Similarly, that could be relaxed in the future if people really need
> the vector crypto acceleration on 32-bit CPUs...  But similarly, the code would
> need to be revised and tested in that configuration.
> 
> > Eric/Jerry (although read the previous paragraph too):
> > I noticed that the sha256 glue code calls crypto_simd_usable(), and in
> > turn may_use_simd() before kernel_vector_begin(). The chacha20 glue code
> > does not call either, which seems to violate the edict in
> > kernel_vector_begin()'s kerneldoc:
> > "Must not be called unless may_use_simd() returns true."
> 
> skcipher algorithms can only be invoked in process and softirq context.  This
> differs from shash algorithms which can be invoked in any context.
> 
> My understanding is that, like arm64, RISC-V always allows non-nested
> kernel-mode vector to be used in process and softirq context -- and in fact,
> this was intentionally done in order to support use cases like this.  So that's
> why the RISC-V skcipher algorithms don't check for may_use_simd() before calling
> kernel_vector_begin().

I see, thanks for explaining that. I think you should probably check
somewhere if has_vector() returns true in that driver though before
using vector instructions. Only checking vlen seems to me like relying on
an implementation detail and if we set vlen for the T-Head/0.7.1 vector
it'd be fooled. That said, I don't think that any of the 0.7.1 vector
systems actually support Zvkb, but I hope you get my drift.

Thanks,
Conor.

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