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Date:   Mon, 10 Feb 2020 18:03:28 +0000
From:   Mark Rutland <mark.rutland@arm.com>
To:     Will Deacon <will@kernel.org>
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: <20200210180327.GB20840@lakrids.cambridge.arm.com>
References: <20191018161033.261971-1-samitolvanen@google.com>
 <20200128184934.77625-1-samitolvanen@google.com>
 <20200128184934.77625-10-samitolvanen@google.com>
 <6f62b3c0-e796-e91c-f53b-23bd80fcb065@arm.com>
 <20200210175214.GA23318@willie-the-truck>
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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.

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).

Thanks,
Mark.