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

[Ковалёв Сергей <valor@xxxxxxx>]

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.

> With these perspectives in mind I don't think this would be a useful 
> addition. Please prove me wrong with performance numbers and a specific 
> use-case that warrants adding this and how you plan to introduce it into 
> LibVMI without causing performance regression to all other use-cases.
I will send You a PR into LibVMI in a day or two. I don't have any
performance numbers at the time. I send this patch to share my current
work as soon as possible.

To prevent regression in all use-cases we could add a configure option.
Thanks to make me notice that!

> Tamas

--
С уважением,
Ковалёв Сергей.