On 13.08.2024 14:40, Alejandro Vallejo wrote: > On Mon Aug 12, 2024 at 4:23 PM BST, Jan Beulich wrote: >> On 08.08.2024 15:41, Alejandro Vallejo wrote: >>> --- a/xen/arch/x86/hvm/hvm.c >>> +++ b/xen/arch/x86/hvm/hvm.c >>> @@ -1164,10 +1164,25 @@ static int cf_check hvm_load_cpu_ctxt(struct domain >>> *d, hvm_domain_context_t *h) >>> seg.attr = ctxt.ldtr_arbytes; >>> hvm_set_segment_register(v, x86_seg_ldtr, &seg); >>> >>> - /* Cover xsave-absent save file restoration on xsave-capable host. */ >>> - vcpu_setup_fpu(v, xsave_enabled(v) ? NULL : v->arch.xsave_area, >>> - ctxt.flags & XEN_X86_FPU_INITIALISED ? ctxt.fpu_regs : >>> NULL, >>> - FCW_RESET); >>> + /* >>> + * On Xen 4.1 and later the FPU state is restored on later HVM context >>> in >>> + * the migrate stream, so what we're doing here is initialising the FPU >>> + * state for guests from even older versions of Xen. >>> + * >>> + * In particular: >>> + * 1. If there's an XSAVE context later in the stream what we do >>> here for >>> + * the FPU doesn't matter because it'll be overriden later. >>> + * 2. If there isn't and the guest didn't use extended states it's >>> still >>> + * fine because we have all the information we need here. >>> + * 3. If there isn't and the guest DID use extended states (could've >>> + * happened prior to Xen 4.1) then we're in a pickle because we >>> have >>> + * to make up non-existing state. For this case we initialise the >>> FPU >>> + * as using x87/SSE only because the rest of the state is gone. >> >> Was this really possible to happen? Guests wouldn't have been able to >> turn on CR4.OSXSAVE, would they? > > You may be right, but my reading of the comment and the code was that > xsave_enabled(v) might be set and the XSAVE hvm context might be missing in > the > stream. The archives didn't shed a lot more light than what the code already > gives away. > > Otherwise it would've been far simpler to unconditionally pass > v->arch.xsave_area to the second parameter and let the xsave area to be > overriden by the follow-up HVM context with its actual state. > > If my understanding is wrong, I'm happy to remove (3), as I don't think it > affects the code anyway. I thought however that it was a relevant data point > to leave paper trail for.
I would certainly agree - as long as it describes (past) reality. If it doesn't, I consider it misleading. Jan
