linux-kernel - RE: [PATCH v2] kvm: x86: mmu: Add cast to negated bitmasks in update_permission

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Date:   Mon, 25 Jun 2018 16:02:40 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Matthias Kaehlcke' <mka@...omium.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joe Perches <joe@...ches.com>
Subject: RE: [PATCH v2] kvm: x86: mmu: Add cast to negated bitmasks in
 update_permission_bitmask()

From: Matthias Kaehlcke
> Sent: 19 June 2018 20:25
> To: Paolo Bonzini; Radim Krčmář; Thomas Gleixner; H . Peter Anvin
> Cc: x86@...nel.org; kvm@...r.kernel.org; linux-kernel@...r.kernel.org; Nick Desaulniers; Joe Perches;
> Matthias Kaehlcke
> Subject: [PATCH v2] kvm: x86: mmu: Add cast to negated bitmasks in update_permission_bitmask()
> 
> 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;
>        ~~                               ^~

Shoot the compiler writer - or turn off that warning.
Or try 'w ^ 0xff'.

	David