Re: [PATCH 0/5] target/arm: Support variable sized coprocessor registers

[Peter Maydell]

On Mon, 11 Apr 2022 at 13:02, Andrew Jones <drjones@redhat.com> wrote:
>
> On Mon, Apr 11, 2022 at 10:22:59AM +0100, Peter Maydell wrote:
> > Also, we support SVE today, and we don't have variable size
> > coprocessor registers. Is there a bug here that we would be
> > fixing ?
>
> SVE registers are KVM_REG_SIZE_U2048 and KVM_REG_SIZE_U256 sized
> registers. They work fine (just like the VFP registers which are
> KVM_REG_SIZE_U128 sized). They work because they don't get stored in the
> cpreg list. SVE and CORE (which includes VFP) registers are filtered
> out by kvm_arm_reg_syncs_via_cpreg_list(). Since they're filtered
> out they need to be handled specifically by kvm_arch_get/put_registers()

Right, this is the distinction between ONE_REG registers and
coprocessor registers (which are a subset of ONE_REG registers).
We wouldn't want to handle SVE regs in the copro list anyway,
I think, because we want their state to end up in env->vfp.zregs[]
so the gdbstub can find it there. And we wouldn't have benefited
from the copro regs handling's "no need for new QEMU to handle
migrating state of a new register" because we needed a lot of
special case code for SVE and couldn't enable it by default
for other reasons.

If we do add non-64-bit cpregs on the kernel side then we need to
make those new registers opt-in, because currently deployed QEMU
will refuse to start if the kernel passes it a register in the
KVM_GET_REG_LIST that is larger than 64 bits and isn't
KVM_REG_ARM_CORE or KVM_REG_ARM64_SVE (assuming I'm not misreading
the QEMU code).

-- PMM