On 04/11/2018 08:10 PM, Lars Kurth wrote:
> ### [PATCH RFC 00/14] EPT-Based Sub-page Write Protection Support
> ### (Zhang Yi)
> RFC posted by Zhang Yi Oct 19, 2017
> No acks, reviews only by memaccess maintainers / developers
> Issues: Use case for the feature is still not clear and needs discussion
> Lars: who needs to review?
> Andrew: mainly George, Tamas, Razvan - major changes to ept2pm structure
> page table structure) and Andrew.
> Andrew: did take a quick look. Nothing egregious in it. Did not have a lot of
> free time to look
> at it.
> George: was going to look at it, but focus on NVDIMM first and EPT stuff
> Tamas: provides write protection on the sub-page - my tools don't have a
> use-case for this.
> Also not sure how this would integrate into alt2pm. Razvan may have a
> Andrew: no interaction with alt2pm, write protection 128 byte aligned
> granularity instead of
> 4K for write tracking a substructure within a page
> Zhang Yi: I have a change in the patch which he wants to look at
> George: can look at modifying that or store the type information elsewhere
> Andrew: Have to shuffle the bits in the EPT tree
> Summary: the issue isn’t really about use-cases, but primarily about
> prioritizing George’s
> **ACTION (April):** Andrew will poke Razvan (give some indication interface
> ios going)
> **ACTION (April):** George will pick this up after NVDIMM has made some
Indeed we do have use cases for this and are very interested in the
feature. As you might expect, this is about optimizing: in several cases
we're only interested in a narrow part of a page, but need to set
restrictions on 4096 bytes - this causes needless page fault exits /
It would indeed need to play well with altp2m to be as useful as possible.
Our use case for altp2m (assuming a stable altp2m - I'm still debugging
the logdirty VGA issue) is to use it for 1) the cases where the emulator
doesn't know an instruction that has caused a page fault (in which case
we get an UNIMPLEMENTED vm_event), and 2) as a workaround for a
interrupt race issue that can happen with emulation.
2) is hopefully a temporary problem, 1) is probably not - new
instructions do appear with new hardware.
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