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Thu, 19 Feb 2026 07:42:13 -0800 (PST) Message-ID: Date: Thu, 19 Feb 2026 16:42:11 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Content-Language: en-US To: "xen-devel@lists.xenproject.org" Cc: Andrew Cooper , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= From: Jan Beulich Subject: [PATCH] x86/bitops: adjust partial first word handling in __find_next{,_zero}_bit() 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 Content-Transfer-Encoding: quoted-printable X-ZohoMail-DKIM: pass (identity @suse.com) X-ZM-MESSAGEID: 1771515766853154100 Content-Type: text/plain; charset="utf-8" There's no need to subtract "bit" in what is passed to __scanbit(), as the other bits are zero anyway after the shift (in __find_next_bit()) or can be made so (in __find_next_zero_bit()) by flipping negation and shift. (We actually leverage the same facts in find_next{,_zero}_bit() as well.) This way in __scanbit() the TZCNT alternative can be engaged. Signed-off-by: Jan Beulich Acked-by: Andrew Cooper --- Register allocation (and hence effects on code size of the change here) is pretty "interesting". The compiler doesn't look to realize that while for 64-bit insns it doesn't matter which GPR is used (a REX prefix is needed anyway), 32-bit insns can be helped by preferring the low 8 GPRs. (Granted the inline assembly in __scanbit() may also be a little difficult to deal with.) --- a/xen/arch/x86/bitops.c +++ b/xen/arch/x86/bitops.c @@ -35,8 +35,8 @@ unsigned int __find_next_bit( if ( bit !=3D 0 ) { /* Look for a bit in the first word. */ - set =3D __scanbit(*p >> bit, BITS_PER_LONG - bit); - if ( set < (BITS_PER_LONG - bit) ) + set =3D __scanbit(*p >> bit, BITS_PER_LONG); + if ( set < BITS_PER_LONG ) return (offset + set); offset +=3D BITS_PER_LONG - bit; p++; @@ -85,8 +85,8 @@ unsigned int __find_next_zero_bit( if ( bit !=3D 0 ) { /* Look for zero in the first word. */ - set =3D __scanbit(~(*p >> bit), BITS_PER_LONG - bit); - if ( set < (BITS_PER_LONG - bit) ) + set =3D __scanbit(~*p >> bit, BITS_PER_LONG); + if ( set < BITS_PER_LONG ) return (offset + set); offset +=3D BITS_PER_LONG - bit; p++;