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Tue, 22 Sep 2026 05:27:50 -0700 (PDT) Message-ID: <20c2003f-910b-4e9b-92f4-5298f11f58df@suse.com> Date: Tue, 22 Sep 2026 14:27:48 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 4/6] xen/riscv: make Zihintpause no longer a required extension To: Baptiste Le Duc Cc: xen-devel@lists.xenproject.org, Alistair Francis , Connor Davis , Oleksii Kurochko , Andrew Cooper , Anthony PERARD , Michal Orzel , Julien Grall , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Stefano Stabellini References: <1789032657.8631fc262581453bbf619ec5b2062170.1a08aa7f23b000c4f3@vates.tech> <1789032898.8631fc262581453bbf619ec5b2062170.1a08aaba10f000c4f3@vates.tech> Content-Language: en-US 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: <1789032898.8631fc262581453bbf619ec5b2062170.1a08aaba10f000c4f3@vates.tech> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-purgate-ID: tlsNG-720697/1790080070-F0AA52AC-4313DE70/0/0 X-purgate-type: clean X-purgate-size: 1334 On 10.09.2026 11:34, Baptiste Le Duc wrote: > required_extensions[] panics at boot if Zihintpause is missing, but Xen > never actually depends on it: cpu_relax() only emits the "pause" hint when > the extension is implemented, otherwise it emits `0x0100000F`, a legally > valid FENCE instruction (`FENCE W, 0`) rather than a native NOP. Just that PAUSE's encoding is 0x0100000F. I.e. what is emitted is always the same, and hence discussing the encoding aspect here doesn't help justify the change. NOP or not also doesn't really matter here. The specific hint encoding looks to fall into what prior to Zihintpause would have been covered by "Designated for future standard use", and hence ... > FENCE is > guaranteed by the RISC-V base ISA, so it never raises an illegal > instruction fault. With an empty successor set, it enforces no > memory-ordering constraints and thus architecturally behaves as a NOP. ... there indeed should be no concern for any platform playing by the rules. > Drop it from required_extensions so hardware without Zihintpause > still boots. > > Assisted-by: Claude:claude-opus-5 > Signed-off-by: Baptiste Le Duc > --- > Changes since v1: > - rewrite commit message. I fear another round of re-writing is going to be necessary, sorry. Jan