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SubjectRe: [PATCH v2] kvm: x86: mmu: Add cast to negated bitmasks in update_permission_bitmask()
Hi Paolo,

On Mon, Jun 25, 2018 at 06:05:01PM +0200, Paolo Bonzini wrote:
> On 19/06/2018 21:25, Matthias Kaehlcke wrote:
> > update_permission_bitmask() negates u8 bitmask values and assigns them
> > to variables of type u8. Since the MSB is set in the bitmask values the
> > compiler expands the negated values to int, which then is assigned to
> > an u8 variable. Cast the negated value back to u8.
> >
> > This fixes several warnings like this when building with clang:
> >
> > arch/x86/kvm/mmu.c:4266:39: error: implicit conversion from 'int' to 'u8'
> > (aka 'unsigned char') changes value from -205 to 51 [-Werror,
> > -Wconstant-conversion]
> > u8 wf = (pfec & PFERR_WRITE_MASK) ? ~w : 0;
> > ~~ ^~
> >
> > (gcc also raises a warning (see https://godbolt.org/g/6JWfWk), however it
> > doesn't seem to be universally enabled)
> >
> > Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
> > Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> > ---
> > Changes in v2:
> > - negate the bitmask at initialization and rename variables to not_X
>
> The patch is not that bad, but I'd like to get confirmation that other
> maintainers are applying fixes like this. Honestly I'm not really
> impressed by most new clang warnings, these days.

Some other instances of the warning that have been addressed are:

commit 644d87dccdc69cf79834a72ed0c889580d6af32a
Author: Stefan Agner <stefan@agner.ch>
Date: Thu Apr 5 16:25:38 2018 -0700

mm/memblock.c: cast constant ULLONG_MAX to phys_addr_t


commit e2a5dca753d1cdc3212519023ed8a13e13f5495b
Author: Borislav Petkov <bp@suse.de>
Date: Thu Nov 23 10:19:51 2017 +0100

x86/umip: Fix insn_get_code_seg_params()'s return value


commit dae1a432ab1fe79ae53129ededeaece35a2dc14d
Author: Nick Desaulniers <nick.desaulniers@gmail.com>
Date: Sat Jun 24 22:50:12 2017 -0700

Input: mousedev - fix implicit conversion warning


commit d1600401faad4bc186bfdb291d8af644465e20bd
Author: Matthias Kaehlcke <mka@chromium.org>
Date: Fri Mar 31 18:00:04 2017 -0700

ALSA: hda/ca0132: Limit values for chip addresses to 32-bit


commit a45463cbf3f9dcdae683033c256f50bded513d6a
Author: Arnd Bergmann <arnd@arndb.de>
Date: Wed Feb 1 18:01:17 2017 +0100

workqueue: avoid clang warning


Thanks

Matthias

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