Re: [Xen-devel] [PATCH 10/10] p2m/ept: Enable PML in p2m-ept for log-dirty.

[Tim Deegan <tim@xxxxxxx>]

At 16:44 +0800 on 10 Apr (1428684271), Kai Huang wrote:
> 
> 
> On 04/09/2015 08:20 PM, Tim Deegan wrote:
> > At 10:35 +0800 on 27 Mar (1427452554), Kai Huang wrote:
> >> @@ -118,6 +119,12 @@ static void ept_p2m_type_to_flags(ept_entry_t *entry, 
> >> p2m_type_t type, p2m_acces
> >>               break;
> >>           case p2m_ram_rw:
> >>               entry->r = entry->w = entry->x = 1;
> >> +            /*
> >> +             * This is about to avoid unnecessary GPA logging in PML 
> >> buffer,
> >> +             * such as normal memory in partial log-dirty
> >> +             */
> >> +            if ( vmx_domain_pml_enabled(p2m->domain) )
> >> +                entry->d = 1;
> > I think that both A and D should be set here, even when PML is not
> > supported, to avoid the MMU having to write them later.  And indeed
> > not just for ram_rw, but for every present leaf type.
> I do agree we should also set A if PML is enabled for the domain, and 
> looks there's no harm to set A bit for all present leaf types. Actually 
> there should be no harm to set A bit even the leaf entry is not present..
> 
> But for D bit I think we should be more specific. For p2m types that are 
> writable, we should set the D-bit to avoid unnecessary GPA logging, but 
> for those are read-only, I think it's not reasonable to set D bit, as 
> it's not possible for them to be dirty.

Fair enough.

> > You will still need to check whether the CPU supports the A/D bits,
> > since AIUI those are reserved bits otherwise.
> In my understanding, A/D bits actually are ignored if EPT A/D bit is not 
> enabled in EPTP. Citing one statement in SDM for an example:
> 
> "If bit 6 of EPTP is 1, accessed flag for EPT; indicates whether 
> software has accessed the 4-KByte page referenced
> by this entry (see Section 28.2.4). Ignored if bit 6 of EPTP is 0".

I see.  I think I was confused by the field being called "reserved" in
the struct.  I've just checked an older copy of the SDM and those bits
used to be "Ignored", so we can just set them on all hardware.  Good!

Tim.

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