Re: [PATCH v7 08/13] KVM: selftests: Use the right memslot for code, page-tables, and data allocations

Ricardo Koller
2022-09-20T18:57:22Z

On Tue, Sep 20, 2022 at 06:40:03PM +0000, Sean Christopherson wrote:
> On Tue, Sep 20, 2022, Ricardo Koller wrote:
> > On Tue, Sep 20, 2022 at 06:07:13PM +0000, Sean Christopherson wrote:
> > > On Tue, Sep 20, 2022, Ricardo Koller wrote:
> > > > The previous commit added support for callers of ____vm_create() to specify
> > >
> > > Changelog is stale, ____vm_create() no longer takes the struct.
> > >
> > > Side topic, it's usually a good idea to use "strong" terminology when referencing
> > > past/future changes, e.g. if patches get shuffled around for whatever reason,
> > > then "previous commit" may become stale/misleading.
> > >
> > > It's fairly easy to convey the same info ("something happened recently" or
> > > "something is going to happen soon") without being so explicit, e.g.
> > >
> > > Wire up common code to use the appropriate code, page table, and data
> > > memmslots that were recently added instead of hardcoding '0' for the
> > > memslot.
> > >
> > > or
> > >
> > > Now that kvm_vm allows specifying different memslots for code, page
> > > tables, and data, use the appropriate memslot when making allocations
> > > in common/libraty code.
> > >
> > > > what memslots to use for code, page-tables, and data allocations. Change
> > > > them accordingly:
> > > >
> > > > - stacks, code, and exception tables use the code memslot
> > >
> > > Huh? Stacks and exceptions are very much data, not code.
> > >
> >
> > I would *really* like to have the data region only store test data. It
> > makes things easier for the test implementation, like owning the whole
> > region.
>
> That's fine, but allocating stack as "code" is super confusing.
>
> > At the same I wanted to have a single region for all the "core pages" like
> > code, descriptors, exception tables, stacks, etc. Not sure what to call it
> > though.
>
> Why? Code is very different than all those other things. E.g. the main reason
> KVM doesn't provide "not-executable" or "execute-only" memslots is because there's
> never been a compelling use case, not because it's difficult to implement. If KVM
> were to ever add such functionality, then we'd want/need selftests to have a
> dedicated code memslot.
>
> > So, what about one of these 2 options:
> >
> > Option A: 3 regions, where we call the "code" region something else, like
> > "core".
> > Option B: 4 regions: code, page-tables, core-data (stacks, exception tables, etc),
> > test-data.
>
> I like (B), though I'd just call 'em "DATA" and "TEST_DATA". IIUC, TEST_DATA is
> the one you want to be special, i.e. it's ok if something that's not "core" allocates
> in DATA, but it's not ok if "core" allocates in TEST_DATA. That yields an easy
> to understand "never use TEST_DATA" rule for library/common/core functionality,
> with the code vs. page tables vs. data decision (hopefully) being fairly obvious.
>
> Defining CORE_DATA will force developers to make judgement calls and probably
> lead to bikeshedding over whether something is considered "core" code.

Sounds good, Option B then (with code, pt, data, test-data).

Thanks,
Ricardo