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

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