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

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Date:   Mon, 10 Feb 2020 18:07:41 +0000
From:   Will Deacon <will@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     James Morse <james.morse@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 09/11] arm64: disable SCS for hypervisor code

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.

Will