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

Sean Christopherson
2022-09-20T18:40:12Z

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.