arm64/fpsimd: Clarify the purpose of using last in fpsimd_save()

When saving the floating point context in fpsimd_save() we always reference
the state using last-> rather than using current->. Looking at the FP code
in isolation the reason for this is not entirely obvious, it's done because
when KVM is running it will bind the guest context and rely on the host
writing out the guest state on context switch away from the guest.

There's a slight trick here in that KVM still uses TIF_FOREIGN_FPSTATE and
TIF_SVE to communicate what needs to be saved, it maintains those flags
and restores them when it is done running the guest so that the normal
restore paths function when we return back to userspace.

Add a comment to explain this to help future readers work out what's going
on a bit faster.

Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220124161115.115200-1-broonie@kernel.org

arch/arm64/kernel/fpsimd.c

@@ -348,7 +348,13 @@ static void task_fpsimd_load(void)
 
 /*
  * Ensure FPSIMD/SVE storage in memory for the loaded context is up to
- * date with respect to the CPU registers.
+ * date with respect to the CPU registers. Note carefully that the
+ * current context is the context last bound to the CPU stored in
+ * last, if KVM is involved this may be the guest VM context rather
+ * than the host thread for the VM pointed to by current. This means
+ * that we must always reference the state storage via last rather
+ * than via current, other than the TIF_ flags which KVM will
+ * carefully maintain for us.
  */
 static void fpsimd_save(void)
 {