On Mon, Apr 11, 2022 at 10:22:59AM +0100, Peter Maydell wrote:
On Mon, 11 Apr 2022 at 07:59, Gavin Shan <gshan@redhat.com> wrote:
There are two arrays for each CPU, to store the indexes and values of the
coprocessor registers. Currently, 8 bytes fixed storage space is reserved
for each coprocessor register. However, larger coprocessor registers have
been defined and exposed by KVM. Except SVE registers, no coprocessor
register exceeds 8 bytes in size. It doesn't mean large coprocessor registers
won't be exploited in future. For example, I'm looking into SDEI virtualization
support, which isn't merged into Linux upstream yet. I have plan to add
several coprocessor ("firmware pseudo") registers to assist the migration.
So, can you give an example of coprocessor registers which are
not 8 bytes in size? How are they accessed by the guest?
If we need to support them then we need to support them, but this
cover letter/series doesn't seem to me to provide enough detail
to make the case that they really are necessary.
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()
I asked Gavin to check if following the SVE pattern made sense for his
use case prior to sending this series, but I don't see the rationale for
not following the SVE pattern in this cover letter. Gavin, can you please
explain why following the SVE pattern doesn't work? Or, are you trying to
avoid adding an additional special case?