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Date:   Tue, 11 Feb 2020 09:54:02 +0000
From:   Will Deacon <will@kernel.org>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     James Morse <james.morse@arm.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Dave Martin <Dave.Martin@arm.com>,
        Kees Cook <keescook@chromium.org>,
        Laura Abbott <labbott@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Jann Horn <jannh@google.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        clang-built-linux@googlegroups.com,
        kernel-hardening@lists.openwall.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v7 09/11] arm64: disable SCS for hypervisor code
Message-ID: <20200211095401.GA8560@willie-the-truck>
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On Mon, Feb 10, 2020 at 06:24:32PM +0000, Mark Rutland wrote:
> On Mon, Feb 10, 2020 at 06:07:41PM +0000, 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?
> 
> That covers the hyp->guest->hyp round trip, but not the host->hyp->host
> portion surrounding that.

Thanks, I missed that. It's annoying that we'll end up needing /both/
-ffixed-x18 *and* the save/restore around guest transitions, but if we
actually want to use SCS for the VHE code then I see that it will be
required.

Sami -- can you restore -ffixed-x18 and then try the function attribute
as suggested by James, please?

Will