[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v4] x86: add p2m_mmio_write_dm
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.) 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. _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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