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, <t...@xen.org> 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.)
Thank you, Tim & Jan.
To give a summary for all the comments:
1> new p2m type p2m_mmio_write_dm is to be added;
2> new p2m type need to be added to P2M_RO_TYPES class;
3> new p2m class, say P2M_DISCARD_WRITE_TYPES(which only include
p2m_ram_ro and p2m_grant_map_ro), and the new predicates, say
p2m_is_discard_write are needed to in these 4 places to discard the
write op;
4> and of cause hvm_hap_nested_page_fault() do not need the special
handling for p2m_grant_map_ro anymore;
5> coding style changes pointed out by Jan
6> clear the commit message
I'll prepare the patch and thanks! :)
Yu
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.
- 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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