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Thu, 14 Mar 2024 07:16:15 -0700 (PDT) Message-ID: <76a1e81d-86d1-438c-975e-c4db46af93ee@suse.com> Date: Thu, 14 Mar 2024 15:16:13 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/7] xen/bitops: Implement ffs() in common logic Content-Language: en-US To: Andrew Cooper Cc: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Wei Liu , Stefano Stabellini , Julien Grall , Volodymyr Babchuk , Bertrand Marquis , Michal Orzel , Oleksii Kurochko , Shawn Anastasio , "consulting @ bugseng . com" , Simone Ballarin , Federico Serafini , Nicola Vetrini , Xen-devel References: <20240313172716.2325427-1-andrew.cooper3@citrix.com> <20240313172716.2325427-3-andrew.cooper3@citrix.com> From: Jan Beulich Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: <20240313172716.2325427-3-andrew.cooper3@citrix.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 13.03.2024 18:27, Andrew Cooper wrote: > --- a/xen/arch/arm/include/asm/bitops.h > +++ b/xen/arch/arm/include/asm/bitops.h > @@ -157,7 +157,7 @@ static inline int fls(unsigned int x) > } > > > -#define ffs(x) ({ unsigned int __t = (x); fls(ISOLATE_LSB(__t)); }) > +#define arch_ffs(x) ({ unsigned int __t = (x); fls(ISOLATE_LSB(__t)); }) The way the macro is invoked, I don't think the helper local variable is then needed anymore? > --- a/xen/arch/x86/include/asm/bitops.h > +++ b/xen/arch/x86/include/asm/bitops.h > @@ -430,16 +430,23 @@ static inline int ffsl(unsigned long x) > return (int)r+1; > } > > -static inline int ffs(unsigned int x) > +static inline unsigned int arch_ffs(unsigned int x) > { > - int r; > + int r = -1; > + > + /* > + * The AMD manual states that BSF won't modify the destination register if > + * x=0. The Intel manual states that the result is undefined, but the > + * architects have said that the register is written back with it's old > + * value, possibly zero extended above 32 bits. > + */ > + asm ( "bsf %[val], %[res]" > + : [res] "+r" (r) > + : [val] "rm" (x) ); And this isn't what the compiler would be doing anyway? Also, just to mention it: I take it that you/we are sure that disallowing both operands to be the same register is still better than ... > - asm ( "bsf %1,%0\n\t" > - "jnz 1f\n\t" > - "mov $-1,%0\n" > - "1:" : "=r" (r) : "rm" (x)); ... the original form? > --- a/xen/common/bitops.c > +++ b/xen/common/bitops.c > @@ -34,8 +34,18 @@ > RUNTIME_CHECK(fn, val, res); \ > } while ( 0 ) > > +static void test_ffs(void) Nit: __init please, even if there ought to be no reason for the compiler to not inline this function. > --- a/xen/include/xen/bitops.h > +++ b/xen/include/xen/bitops.h > @@ -110,6 +110,21 @@ static inline int generic_flsl(unsigned long x) > > #include > > +/* > + * Find First Set bit. Bits are labelled from 1. > + */ > +static always_inline __pure unsigned int ffs(unsigned int x) Why always_inline? > +{ > + if ( __builtin_constant_p(x) ) > + return __builtin_ffs(x); > + > +#ifndef arch_ffs > +#define arch_ffs __builtin_ffs > +#endif > + > + return arch_ffs(x); > +} Just to mention it: __builtin_ffs() takes and returns plain int. I'm happy about our own helper being unsigned-correct, but anything like this has a Misra angle too. Jan