Re: [PATCH v3 03/19] target/arm: Restrict DC-CVAP instruction to TCG accel

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

On Mon, 16 Mar 2020 at 19:36, Richard Henderson
<richard.henderson@linaro.org> wrote:
> I'm not 100% sure how the system regs function under kvm.
>
> If they are not used at all, then we should avoid them all en masse an not
> piecemeal like this.
>
> If they are used for something, then we should keep them registered and change
> the writefn like so:
>
> #ifdef CONFIG_TCG
>     /* existing stuff */
> #else
>     /* Handled by hardware accelerator. */
>     g_assert_not_reached();
> #endif

(1) for those registers where we need to know the value within
QEMU code (notably anything involved in VA-to-PA translation,
as this is used by gdbstub accesses, etc, but sometimes we
want other register values too): the sysreg struct is
what lets us map from the KVM register to the field in the
CPU struct when we do a sync of data to/from the kernel.

(2) for other registers, the sync lets us make the register
visible as an r/o register in the gdbstub. (this is not
very important, but it's nice)

(3) Either way, the sync works via the raw_read/raw_write
accessors (this is a big part of what they're for), which are
supposed to just stuff the data into/out of the underlying
CPU struct field. (But watch out because we fall back to
using the non-raw read/writefn if there's no raw version
provided for a particular register.) If a regdef is marked
as NO_RAW then it means there is no raw access and we don't
sync the value.

(4) I think that in KVM mode we won't deliberately do
non-raw accesses, and a quick grep through of the places
that do 'readfn' accesses supports that.

thanks
-- PMM