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Re: [PATCH v2 1/3] arm64/sve: Fix missing SVE/FPSIMD endianness conversions
- From: Will Deacon <will dot deacon at arm dot com>
- To: Dave Martin <Dave dot Martin at arm dot com>
- Cc: linux-arm-kernel at lists dot infradead dot org, gdb at sourceware dot org, Alex Bennée <alex dot bennee at linaro dot org>, Julien Grall <julien dot grall at arm dot com>, Catalin Marinas <catalin dot marinas at arm dot com>, Peter Maydell <peter dot maydell at linaro dot org>, Zhang Lei <zhang dot lei at jp dot fujitsu dot com>, Alan Hayward <alan dot hayward at arm dot com>
- Date: Wed, 12 Jun 2019 18:28:53 +0100
- Subject: Re: [PATCH v2 1/3] arm64/sve: Fix missing SVE/FPSIMD endianness conversions
- References: <1560355234-25516-1-git-send-email-Dave.Martin@arm.com> <1560355234-25516-2-git-send-email-Dave.Martin@arm.com>
On Wed, Jun 12, 2019 at 05:00:32PM +0100, Dave Martin wrote:
> The in-memory representation of SVE and FPSIMD registers is
> different: the FPSIMD V-registers are stored as single 128-bit
> host-endian values, whereas SVE registers are stored in an
> endianness-invariant byte order.
>
> This means that the two representations differ when running on a
> big-endian host. But we blindly copy data from one representation
> to another when converting between the two, resulting in the
> register contents being unintentionally byteswapped in certain
> situations. Currently this can be triggered by the first SVE
> instruction after a syscall, for example (though the potential
> trigger points may vary in future).
>
> So, fix the conversion functions fpsimd_to_sve(), sve_to_fpsimd()
> and sve_sync_from_fpsimd_zeropad() to swab where appropriate.
>
> There is no common swahl128() or swab128() that we could use here.
> Maybe it would be worth making this generic, but for now add a
> simple local hack.
>
> Since the byte order differences are exposed in ABI, also clarify
> the docuentation.
>
> Fixes: bc0ee4760364 ("arm64/sve: Core task context handling")
> Fixes: 8cd969d28fd2 ("arm64/sve: Signal handling support")
> Fixes: 43d4da2c45b2 ("arm64/sve: ptrace and ELF coredump support")
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Thanks, Dave. I've picked this one up and pushed it out to our fixes branch
for 5.2. I assume Catalin will take the other two for 5.3. It's probably
worth proposing some generic 128-bit swab functions too, in case there's any
interest (in which case we could drop our local implementation later on).
Cheers,
Will