Re: [XEN PATCH v1 1/1] x86/domctl: add gva_to_gfn command

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Mon, Mar 20, 2023 at 3:23 PM Ковалёв Сергей <valor@xxxxxxx> wrote:
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>
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> 21.03.2023 1:51, Tamas K Lengyel wrote:
> >
> >
> > On Mon, Mar 20, 2023 at 12:32 PM Ковалёв Сергей <valor@xxxxxxx
> > <mailto:valor@xxxxxxx>> wrote:
> >  >
> >  > gva_to_gfn command used for fast address translation in LibVMI project.
> >  > With such a command it is possible to perform address translation in
> >  > single call instead of series of queries to get every page table.
> >
> > You have a couple assumptions here:
> >   - Xen will always have a direct map of the entire guest memory - there
> > are already plans to move away from that. Without that this approach
> > won't have any advantage over doing the same mapping by LibVMI
>
> Thanks! I didn't know about the plan. Though I use this patch
> back ported into 4.16.
>
> >   - LibVMI has to map every page for each page table for every lookup -
> > you have to do that only for the first, afterwards the pages on which
> > the pagetable is are kept in a cache and subsequent lookups would be
> > actually faster then having to do this domctl since you can keep being
> > in the same process instead of having to jump to Xen.
>
> Yes. I know about the page cache. But I have faced with several issues
> with cache like this one https://github.com/libvmi/libvmi/pull/1058 .
> So I had to disable the cache.

The issue you linked to is an issue with a stale v2p cache, which is a virtual TLB. The cache I talked about is the page cache, which is just maintaining a list of the pages that were accessed by LibVMI for future accesses. You can have one and not the other (ie. ./configure  --disable-address-cache --enable-page-cache).

Tamas