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([2001:b07:6468:f312:d0d9:ea10:9775:f33f]) by smtp.gmail.com with ESMTPSA id n2sm12547831wro.96.2020.02.28.03.12.52 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 28 Feb 2020 03:12:52 -0800 (PST) Subject: Re: [kvm-unit-tests PATCH 5/7] svm: convert neg shift to unsigned shift To: Bill Wendling , kvm@vger.kernel.org Cc: oupton@google.com, drjones@redhat.com References: <20200226074427.169684-1-morbo@google.com> <20200226094433.210968-1-morbo@google.com> <20200226094433.210968-10-morbo@google.com> From: Paolo Bonzini Message-ID: Date: Fri, 28 Feb 2020 12:12:52 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.4.1 MIME-Version: 1.0 In-Reply-To: <20200226094433.210968-10-morbo@google.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 26/02/20 10:44, Bill Wendling wrote: > Shifting a negative signed value is undefined. Use a shift of an > unsigned value instead. > > Signed-off-by: Bill Wendling In practice only UBSAN complains; GCC even documents that it does not treat it as undefined (which would be insane, especially if the multiplication by 2 does not overflow as in this case): GCC does not use the latitude given in C99 only to treat certain aspects of signed '<<' as undefined, but this is subject to change. But in the interest of purity we should instead compile with -fwrapv. Paolo > --- > x86/svm.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/x86/svm.c b/x86/svm.c > index ae85194..17be4b0 100644 > --- a/x86/svm.c > +++ b/x86/svm.c > @@ -1148,7 +1148,7 @@ static bool npt_rw_l1mmio_check(struct test *test) > } > > #define TSC_ADJUST_VALUE (1ll << 32) > -#define TSC_OFFSET_VALUE (-1ll << 48) > +#define TSC_OFFSET_VALUE (~0ull << 48) > static bool ok; > > static void tsc_adjust_prepare(struct test *test) >