On Mon, 8 Jan 2024, Carlo Nonato wrote: > Hi Jan, > > On Mon, Jan 8, 2024 at 5:40 PM Jan Beulich <jbeul...@suse.com> wrote: > > > > On 08.01.2024 17:31, Carlo Nonato wrote: > > > On Mon, Jan 8, 2024 at 4:32 PM Julien Grall <jul...@xen.org> wrote: > > >> On 08/01/2024 15:18, Carlo Nonato wrote: > > >>>> No. I am saying that that we should not be able to allow changing the > > >>>> colors after the memory has been allocated. To give an example, your > > >>>> current code would allow: > > >>>> > > >>>> 1) Setup the P2M pools or allocate RAM > > >>>> 2) Set the color > > >>>> > > >>>> This would render the coloring configuration moot. > > >>>> > > >>>> Whether we want to allow changing the coloring before hand is a > > >>>> different question and as I wrote earlier on, I don't mind if you want > > >>>> to forbid that. > > >>> > > >>> At the moment I'm relying on the toolstack in the sense that I know > > >>> that it > > >>> will set colors right after domain creation and before memory > > >>> allocation. > > >>> Calling alloc_domheap_pages() without a coloring configuration makes Xen > > >>> crash, so it's mandatory to have the configuration done before any > > >>> allocation. > > >>> I know that we shouldn't rely on the toolstack this much, but I didn't > > >>> find a better way. Given this assumption, looking for an already > > >>> existing > > >>> color configuration of a domain is sufficient to avoid what you are > > >>> saying. > > >>> > > >>> Is it possible to enforce such a constraint with domctl? > I mean to be > > >>> sure that this domctl will be called at a precise time. > > >> > > >> Yes. You can... > > >> > > >>> > > >>> Thanks. > > >>> > > >>>>> I don't know what to check that. > > >>>> > > >>>> You can check the size of the P2M pool (d->arch.paging.p2m_total_pages) > > >>>> is still 0. I think for RAM, you can check d->tot_pages == 0. > > >> > > >> ... reject the call if either of the two fields are not zero. > > > > > > What I'm saying is that Xen would crash before even reaching this point > > > if no > > > colors were provided. Let's say that the toolstack or whatever hypercall > > > user > > > isn't calling this domctl at all (or not at the right time), then the > > > domain > > > colored allocator would always return null pages since there are no > > > colors. > > > We would have a crash and your if (or mine) would be useless. > > > > Why is it that you can't simply allocated arbitrary memory if coloring > > information wasn't set up front? Aiui that'll be required anyway, as > > there shouldn't be a restriction that all domains have to use coloring. > > If coloring is enabled all domains are colored. It's one of our first > assumptions. We haven't developed something that works hybridly and supporting > that would require some rework.
I think that's a good assumption and I wouldn't go in the direction of supporting a mix of colored and non-colored. To benefit from cache coloring, all domains need to be colored, otherwise a single non-colored domain could thrash the cache of everyone else.
