Re: unit tests and get_user_pages_ptes_fast()

[Andrea Arcangeli <aarcange@redhat.com>]

Hi Avi,

On Mon, Oct 04, 2010 at 11:35:28AM +0200, Avi Kivity wrote:
>   During the last kvm forum, I described a unit test framework that can 
> help test the kvm APIs.  Briefly, it starts a process in host userspace, 
> which sets up a memory slot mapping gpa 0:3G to hva 0:3G.  It then sets 
> up guest registers for unpaged protected mode (or paged protected mode 
> with 1:1 mapping).  The effect is that we have a 1:1 gva->hva 
> translation, and can use KVM_RUN to run host code in guest mode.
> 
> There is a snag however.  kvm calls get_user_pages_fast(.write = 1), and 
> the host process maps its code pages read-only.
> 
> The way I'd like to work around this is to map read-only accesses to 
> read-only pages as read-only.  This also prevents ksm cow pages from 
> being broken by read accesses.  It can also be used to get the page size 
> for transparent huge pages (and later hugetlbfs too).
> 
> So, for a read fault we do:
> 
>    pte_t pte;
>    get_user_pages_ptes_fast(..., page, &pte, 1, .write = 0)
>    ...
>    if (pte_transhuge(pte)) // or however it's called
>        ...
>    ...
>    if (pte_write(pte))
>          map writeable
>    else
>          map readonly
> 
> Any snags?  or alternatives?

I think we tried to do this write=0 before, it provides benefits for
swapins from swapcache too (after the page is unmapped but the
swapcache is present and uptodate and clean). If I recall correctly
the only obstacle was the fact the out of sync code wants to map the
spte writable if it can, but if we use write=0 in gup, it won't know
if it can. If it'd wait a real write access from guest, we'd risk
creating a spte wrprotect fault for nothing (it should only write a
write access from the guest if the linux VM solves the page fault with
a readonly pte, even the minor-read-fault from exclusive swapcache
will not set the dirty bit but it will still set the pte_write on the
pte so the out of sync code should take advantage of that information
in the host pte). In the past you suggested some TRY_WRITE operation
instead of only read/write. We just need to pass the write bit of the
pte somehow to the gup caller.

Full agreement that once we use write=0 for read faults your problem
with the debugging userland running in guest mode will have better
chance to work too :).

Andrea