On 03/05/2021 16:39, Andrew Cooper wrote: > We're soon going to need a compressed helper of the same form. > > The size of the uncompressed image is a strictly a property of the highest > user state. This can be calculated trivially with xstate_offsets/sizes, and > is much faster than a CPUID instruction in the first place, let alone the two > XCR0 writes surrounding it. > > Retain the cross-check with hardware in debug builds, but forgo it normal > builds. In particular, this means that the migration paths don't need to mess > with XCR0 just to sanity check the buffer size. > > Signed-off-by: Andrew Cooper <andrew.coop...@citrix.com>
The Qemu smoketests have actually found a bug here. https://gitlab.com/xen-project/patchew/xen/-/jobs/1232118510/artifacts/file/smoke.serial We call into xstate_uncompressed_size() from hvm_register_CPU_save_and_restore() so the previous "xcr0 == 0" path was critical to Xen not exploding on non-xsave platforms. This is straight up buggy - we shouldn't be registering xsave handlers on non-xsave platforms, but the calculation is also wrong (in the safe directly at least) when we use compressed formats. Yet another unexpected surprise for the todo list. ~Andrew
