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Date: Mon, 10 Feb 2020 18:03:28 +0000
From: Mark Rutland <>
To: Will Deacon <>
Cc: James Morse <>,
	Sami Tolvanen <>,
	Catalin Marinas <>,
	Steven Rostedt <>,
	Masami Hiramatsu <>,
	Ard Biesheuvel <>,
	Dave Martin <>,
	Kees Cook <>,
	Laura Abbott <>, Marc Zyngier <>,
	Nick Desaulniers <>,
	Jann Horn <>,
	Miguel Ojeda <>,
	Masahiro Yamada <>,,,,
Subject: Re: [PATCH v7 09/11] arm64: disable SCS for hypervisor code

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).


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