On Tue, Oct 17, 2017 at 03:31:42PM +0100, Dave Martin wrote: > On Tue, Oct 17, 2017 at 01:50:24PM +0200, Christoffer Dall wrote: > > On Tue, Oct 10, 2017 at 07:38:39PM +0100, Dave Martin wrote: > > > Until KVM has full SVE support, guests must not be allowed to > > > execute SVE instructions. > > > > > > This patch enables the necessary traps, and also ensures that the > > > traps are disabled again on exit from the guest so that the host > > > can still use SVE if it wants to. > > > > > > This patch introduces another instance of > > > __this_cpu_write(fpsimd_last_state, NULL), so this flush operation > > > is abstracted out as a separate helper fpsimd_flush_cpu_state(). > > > Other instances are ported appropriately. > > > > I don't understand this paragraph, beginning from ", so this...". > > > > > > From reading the code, what I think is the reason for having to flush > > the SVE state (and mark the host state invalid) is that even though we > > disallow SVE usage in the guest, the guest can use the normal FP state, > > and while we always fully preserve the host state, this could still > > corrupt some additional SVE state not properly preserved for the host. > > Is that correct? > > Yes, that's right: the guest can't touch the SVE-specific registers > Pn/FFR, but FPSIMD accesses to Vn regs cause the high bits of the > corresponding SVE Zn registers to be clobbered. In any case, the > FPSIMD restore done by KVM after guest exit is sufficient to clobber > those bits even if the guest didn't do it. > > This is a band-aid for not making the KVM world switch code properly > SVE-aware yet. > > Does the following wording sound better: > > --8<-- > > On guest exit, high bits of the SVE Zn registers may have been > clobbered as a side-effect the execution of FPSIMD instructions in > the guest. The existing KVM host FPSIMD restore code is not > sufficient to restore these bits, so this patch explicitly marks > the CPU as not containing cached vector state for any task, this > forcing a reload on the next return to userspace. This is an > interim measure, in advance of adding full SVE awareness to KVM. > > Because of the duplication of this operation > (__this_cpu_write(fpsimd_last_state, NULL)), it is factored out as s/it is/is/ (I think) > a new helper fpsimd_flush_cpu_state() to make the purpose clearer. > > -->8-- > > > > > > > As a side effect of this refactoring, a this_cpu_write() in > > > fpsimd_cpu_pm_notifier() is changed to __this_cpu_write(). This > > > should be fine, since cpu_pm_enter() is supposed to be called only > > > with interrupts disabled. > > > > Otherwise the patch itself looks good to me. > > Thanks, let me know about the above wording change though. > Yes, the wording is good and helps a lot. Thanks for writing that. Reviewed-by: Christoffer Dall <christoffer.dall@xxxxxxxxxx>
