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

At 11:17 +0000 on 01 Dec (1417429027), Jan Beulich wrote:
> >>> On 01.12.14 at 11:30, <tim@xxxxxxx> wrote:
> > At 09:32 +0000 on 01 Dec (1417422746), Jan Beulich wrote:
> >> >>> On 01.12.14 at 09:49, <yu.c.zhang@xxxxxxxxxxxxxxx> wrote:
> >> > To my understanding, pages with p2m_ram_ro are not supposed to be 
> >> > modified by guest. So in __hvm_copy(), when p2m type of a page is 
> >> > p2m_ram_rom, no copy will occur.
> >> > However, to our usage, we just wanna this page to be write protected, so 
> >> > that our device model can be triggered to do some emulation. The content 
> >> > written to this page is supposed not to be dropped. This way, if 
> >> > sequentially a read operation is performed by guest to this page, the 
> >> > guest will still see its anticipated value.
> >> 
> >> __hvm_copy() is only a helper function, and doesn't write to
> >> mmio_dm space either; instead its (indirect) callers would invoke
> >> hvmemul_do_mmio() upon seeing HVMCOPY_bad_gfn_to_mfn
> >> returns. The question hence is about the apparent inconsistency
> >> resulting from writes to ram_ro being dropped here but getting
> >> passed to the DM by hvm_hap_nested_page_fault(). Tim - is
> >> that really intentional?
> > 
> > No - and AFAICT it shouldn't be happening.  It _is_ how it was
> > implemented originally, because it involved fewer moving parts and
> > didn't need to be efficient (and after all, writes to entirely missing
> > addresses go to the device model too).
> > 
> > But the code was later updated to log and discard writes to read-only
> > memory (see 4d8aa29 from Trolle Selander).
> > 
> > Early version of p2m_ram_ro were documented in the internal headers as
> > sending the writes to the DM, but the public interface (HVMMEM_ram_ro)
> > has always said that writes are discarded.
> Hmm, so which way do you recommend resolving the inconsistency
> then - match what the public interface says or what the apparent
> original intention for the internal type was? Presumably we need to
> follow the public interface mandated model, and hence require the
> new type to be introduced.

Sorry, I was unclear -- there isn't an inconsistency; both internal
and public headers currently say that writes are discarded and AFAICT
that is what the code does.

But yes, we ought to follow the established hypercall interface, and
so we need the new type.

> > 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".



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