Re: [PATCH v7 09/11] arm64: disable SCS for hypervisor code

[Marc Zyngier]

On 2020-02-11 09:55, Will Deacon wrote:
> On Tue, Feb 11, 2020 at 09:14:52AM +0000, Marc Zyngier wrote:
> > On 2020-02-10 18:07, Will Deacon wrote:
> > > On Mon, Feb 10, 2020 at 06:03:28PM +0000, Mark Rutland wrote:
> > > > On Mon, Feb 10, 2020 at 05:52:15PM +0000, Will Deacon wrote:
> > > > > On Mon, Feb 10, 2020 at 05:18:58PM +0000, James Morse wrote:
> > > > > > On 28/01/2020 18:49, Sami Tolvanen wrote:
> > > > > > > Filter out CC_FLAGS_SCS and -ffixed-x18 for code that runs at a
> > > > > > > different exception level.
> > > > > >
> > > > > > Hmmm, there are two things being disabled here.
> > > > > >
> > > > > > Stashing the lr in memory pointed to by VA won't work transparently at EL2 ... but
> > > > > > shouldn't KVM's C code still treat x18 as a fixed register?
> > > > >
> > > > > My review of v6 suggested dropping the -ffixed-x18 as well, since it's only
> > > > > introduced by SCS (in patch 5) and so isn't required by anything else. Why
> > > > > do you think it's needed?
> > > >
> > > > When EL1 code calls up to hyp, it expects x18 to be preserved across
> > > > the
> > > > call, so hyp needs to either preserve it explicitly across a
> > > > transitions
> > > > from/to EL1 or always preserve it.
> > >
> > > I thought we explicitly saved/restored it across the call after
> > > af12376814a5 ("arm64: kvm: stop treating register x18 as caller save").
> > > Is
> > > that not sufficient?
> > >
> > > > The latter is easiest since any code used by VHE hyp code will need
> > > > x18
> > > > saved anyway (ans so any common hyp code needs to).
> > >
> > > I would personally prefer to split the VHE and non-VHE code so they can
> > > be
> > > compiled with separate options.
> >
> > This is going to generate a lot of code duplication (or at least 
> > object
> > duplication),
> > as the two code paths are intricately linked and splitting them to 
> > support
> > different
> > compilation options and/or calling conventions.
> >
> > I'm not fundamentally opposed to that, but it should come with ways to 
> > still
> > manage it as a unified code base as much as possible, as ways to 
> > discard the
> > unused part at runtime (which should become easy to do once we have 
> > two
> > distinct sets of objects).
>
> Agreed, and I don't want to hold up the SCS patches because of this. 
> I'm
> just nervous about the function attribute because I've only ever had
> terrible experiences with them. Maybe it will work this time (!)

I have the same experience chasing missing __hyp_text attributes. Until 
we
have tooling that picks on this *at compile time*, we'll have to play
wack-a-mole with them...

        M.
--
Jazz is not dead. It just smells funny...