Messages in this thread | | | Date | Tue, 11 Feb 2020 10:00:01 +0000 | From | Marc Zyngier <> | Subject | Re: [PATCH v7 09/11] arm64: disable SCS for hypervisor code |
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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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