Re: [Xen-devel] [PATCH v4] x86: add p2m_mmio_write_dm

["Yu, Zhang" <yu.c.zhang@xxxxxxxxxxxxxxx>]

On 12/2/2014 7:40 PM, Tim Deegan wrote:
> At 15:38 +0800 on 02 Dec (1417531126), Yu, Zhang wrote:
> > On 12/1/2014 8:13 PM, Tim Deegan wrote:
> > > At 11:17 +0000 on 01 Dec (1417429027), Jan Beulich wrote:
> > > > > > > On 01.12.14 at 11:30, <tim@xxxxxxx> wrote:
> > > > > During this bit of archaeology I realised that either this new type
> > > > > should _not_ be made part of P2M_RO_TYPES, or, better, we need a new
> > > > > class of P2M types (P2M_DISCARD_WRITE_TYPES, say) that should be used
> > > > > for these paths in emulate_gva_to_mfn() and __hvm_copy(), containing
> > > > > just p2m_ram_ro and p2m_grant_map_ro.
> > > > And I suppose in that latter case the new type could be made part
> > > > of P2M_RO_TYPES()?
> > > Yes indeed, as P2M_RO_TYPES is defined as "must have the _PAGE_RW bit
> > > clear in their PTEs".
> > Thanks Tim.
> > Following are my understanding of the P2M_RO_TYPES and your comments.
> > Not sure if I get it right. Please correct me if anything wrong:
> > 1> The P2M_RO_TYPES now bears 2 meanings: one is "w bit is clear in the
> > pte"; and another being to discard the write operations;
> > 2> We better define another class to bear the second meaning.
> Yes, that's what I meant.
>
> Answering your other questions in reverse order:
>
> > 2> p2m_grant_map_ro is also supposed to be discarded? Will handling of
> > this type of pages goes into __hvm_copy()/__hvm_clear(), or should?
> I think so, yes.  At the moment we inject #GP when the guest writes to
> a read-only grant, which is OK: the guest really ought to know better.
> But I think we'll probably end up with neater code if we handle
> read-only grants the same way as p2m_ram_ro.
>
> Anyone else have an opinion on the right thing to do here?
>
> > Also some questions for the new p2m class, say P2M_DISCARD_WRITE_TYPES,
> > and the new predicates, say p2m_is_discard_write:
> > 1> You mentioned emulate_gva_to_mfn() and __hvm_copy() should discard
> > the write ops, yet I also noticed many other places using the
> > p2m_is_readonly, or only the "p2mt == p2m_ram_ro" judgement(in
> > __hvm_copy/__hvm_clear). Among all these other places, is there any ones
> > also supposed to use the p2m_is_discard_write?
> I've just had a look through them all, and I can see exactly four
> places that should be using the new p2m_is_discard_write() test:
>
>   - emulate_gva_to_mfn() (Though in fact it's a no-op as shadow-mode
>     guests never have p2m_ram_shared or p2m_ram_logdirty mappings.)
>   - __hvm_copy()
>   - __hvm_clear() and
>   - hvm_hap_nested_page_fault() (where you should also remove the
>     explicit handling of p2m_grant_map_ro below.)
>
> Looking through that turned up a few other oddities, which I'm
> listing here to remind myself to look at them later (i.e. you don't
> need to worry about them for this patch):
>
>   - nsvm_get_nvmcb_page() and nestedhap_walk_L0_p2m() need to handle
>     p2m_ram_logdirty or they might spuriously fail duiring live
>     migration.
>   - __hvm_copy() and __hvm_clear are probably over-strict in their
>     failure to handle grant types.
Hi Tim. Sorry to bother you. :)
I just noticed that in __hvm_copy()/__hvm_clear(), the grant types are 
handled before the p2m_ram_ro - will return HVMCOPY_unhandleable. So if
p2m_is_discard_write() is supposed to replace the handling of 
p2m_ram_ro, handling of p2m_grant_map_ro will still return 
HVMCOPY_unhandleable, before the p2m_is_discard_write() predicate.
Even we move the testing of p2m_is_discard_write() before the handling 
of grant types, it seems not quite clean.
By "over-strict in their failure to handle grant types.", do you also 
mean this?
Thanks
Yu

>   - P2M_UNMAP_TYPES in vmce.c is a mess.  It's not the right place to
>     define this, since it definitely won't be seen my anyone
>     adding a new type, and it already has an 'XXX' comment that says
>     it doesn't cover a lot of cases. :(
>
> I'll have a look at those another time.
>
> Cheers,
>
> Tim.
>
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