[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

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

On 12/1/2014 8:31 PM, Jan Beulich wrote:
On 01.12.14 at 13:13, <tim@xxxxxxx> wrote:
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.

Not for hvm_hap_nested_page_fault() afaict - the forwarding to
DM there contradicts the "writes are discarded" model that other
code paths follow.

Thanks, Jan.
By "inconsistency", do you mean the p2m_ram_ro shall not trigger the handle_mmio_with_translation() in hvm_hap_nested_page_fault()? I'm also a bit confused with the "writes are discarded/dropped" comments in the code. Does this mean writes to the p2m_ram_ro pages should be abandoned without going to the dm, or going to the dm and ignored later? The code seems to be the second one.


Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.