Re: [PATCH] target/arm: Honor HCR_EL2.TID3 trapping requirements

[Peter Maydell <peter.maydell@linaro.org>]

On Mon, 25 Nov 2019 at 17:08, Marc Zyngier <maz@kernel.org> wrote:
>
> On 2019-11-25 16:21, Peter Maydell wrote:
> > Missing .accessfn for ID_AA4PFR2_EL1_RESERVED ?
>
> Indeed, I oversaw that one. I'll fix it and repost it together with
> the extra patches to handle TID1 and TID2.

Given that rc3 (last chance, more or less, for bugfixes for 4.2)
is tomorrow, I propose that I take this patch with the
+              .accessfn = access_aa64idreg,
line for ID_AA64PFR2_EL1_RESERVED squashed in. I think
TID1/TID2 and the MMFR-from-aarch32 parts are less urgent ?

> > These are the AArch64 accessors for the aarch32 MVFR registers,
> > but this trap bit is also supposed to trap aarch32 accesses to
> > the MVFR registers, which are via the vmrs insn. That needs
> > to be done in trans_VMSR_VMRS(); we can do that with a
> > followup patch, since the mechanism there is completely different.
>
> Holy cow! I'm afraid that my newly acquired QEMU-foo is missing
> a few key options. Does it mean we need to generate a trapping
> instruction, as opposed to take the exception on access?

We will need to generate a call to a helper function which
does the access check (and possibly raises an exception).
We can't determine at translate time whether the insn is
going to cause an exception, because the TID3 bit is not one
of the fairly limited set of information which we put into
the TB flags and allow to be baked into the generated code.
(In theory we could add it, but we don't have very many TB
flags so we only do that for cases where the overhead of doing
the check at runtime matters.)

thanks
-- PMM