Re: [Xen-devel] [RFC 0 PATCH 3/3] PVH dom0: construct_dom0 changes

[Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>]

On Tue, 08 Oct 2013 08:43:43 +0100
"Jan Beulich" <JBeulich@xxxxxxxx> wrote:

> >>> On 08.10.13 at 02:52, Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>
> >>> wrote:
> > On Mon, 30 Sep 2013 07:56:30 +0100
> > "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
> > 
> >> >>> On 28.09.13 at 01:03, Mukesh Rathor <mukesh.rathor@xxxxxxxxxx>
> >> >>> wrote:
> >> > On Fri, 27 Sep 2013 08:01:16 +0100
> >> > "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
> > .......
> >> >> >> > @@ -1089,11 +1262,18 @@ int __init construct_dom0(
> >> >> >> >      regs->eip = parms.virt_entry;
> >> >> >> >      regs->esp = vstack_end;
> >> >> >> >      regs->esi = vstartinfo_start;
> >> >> >> > -    regs->eflags = X86_EFLAGS_IF;
> >> >> >> > +    regs->eflags = X86_EFLAGS_IF | 0x2;
> >> >> >> 
> >> >> >> Unrelated change?
> >> >> > 
> >> >> > Nop, we need to make sure the resvd bit is set in eflags
> >> >> > otherwise it won't vmenter (invalid guest state). Should be
> >> >> > harmless for PV, right? Not sure where it does it for PV
> >> >> > before actually scheduling it..
> >> >> 
> >> >> PV doesn't set this anywhere - the hardware doesn't allow the
> >> >> flag to be cleared (writes are ignored). If VMENTER is picky
> >> >> about this, the GUEST_RFLAGS write at the end of
> >> >> vmx_vmenter_helper() should be doing this instead of having to
> >> >> do it here (and obviously in some other place for DomU
> >> >> creation).
> >> > 
> >> > For domU we set it in arch_set_info_guest.
> >> 
> >> Which is bogus too. 15910:ec3b23d8d544 ("hvm: Always keep
> >> canonical copy of RIP/RSP/RFLAGS in guest_cpu_user_regs()") did
> >> this adjustment without really explaining why it can't be done
> >> centrally in just the two places copying regs->eflags into the
> >> VMCS/VMCB spot.
> > 
> > I beg to differ.... such nit picking is equally bogus IMHO. The
> > bit needs to be set once, putting it in vmx_vmenter_helper adds an
> > unnecessary slowdown IMO. 
> 
> An "or" being a measurable slowdown?

Well, unnecessary dirtying of cache line. But, I'll make the change
just to be done with this.

> >> > vmx_vmenter_helper gets
> >> > called on every vmentry, we just need this setting once.
> >> 
> >> Would a debugger update guest state via arch_set_info_guest()?
> >> I doubt it. It would imo be a desirable up front cleanup patch to
> >> move this bogus thing out of arch_set_info_guest() into
> >> vmx_vmenter_helper() (and whatever SVM equivalent, should
> >> SVM too be incapable of dealing with the flag being clear). See
> >> how e.g. hvm_load_cpu_ctxt() already sets the flag? It's really
> >> like being done almost at random...
> > 
> > The debugger would always read eflags, muck with only
> > the bits it needs to, leaving the resvd bit as is, then send it
> > down.
> 
> So you'd expect every debugger to be responsible for setting this
> bit? Pretty odd a requirement, when it can be done centrally in a
> single place, covering all cases.

Like I said, the debuggers are expected to read existing eflags,
change the bits (leaving resvd bit set). But, ir-relevant now
after the above change.

Mukesh


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