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Tue, 14 May 2024 04:57:06 -0700 (PDT) Message-ID: <435ccfc3-d4e3-4da4-838f-4fb0e8987eab@suse.com> Date: Tue, 14 May 2024 13:57:13 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH for-4.19] x86/mtrr: avoid system wide rendezvous when setting AP MTRRs Content-Language: en-US To: Roger Pau Monne Cc: Andrew Cooper , Oleksii Kurochko , xen-devel@lists.xenproject.org References: <20240513085925.59324-1-roger.pau@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: <20240513085925.59324-1-roger.pau@citrix.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 13.05.2024 10:59, Roger Pau Monne wrote: > --- a/xen/arch/x86/cpu/mtrr/main.c > +++ b/xen/arch/x86/cpu/mtrr/main.c > @@ -573,14 +573,15 @@ void mtrr_ap_init(void) > if (!mtrr_if || hold_mtrr_updates_on_aps) > return; > /* > - * Ideally we should hold mtrr_mutex here to avoid mtrr entries changed, > - * but this routine will be called in cpu boot time, holding the lock > - * breaks it. This routine is called in two cases: 1.very earily time > - * of software resume, when there absolutely isn't mtrr entry changes; > - * 2.cpu hotadd time. We let mtrr_add/del_page hold cpuhotplug lock to > - * prevent mtrr entry changes > + * hold_mtrr_updates_on_aps takes care of preventing unnecessary MTRR > + * updates when batch starting the CPUs (see > + * mtrr_aps_sync_{begin,end}()). > + * > + * Otherwise just apply the current system wide MTRR values to this AP. > + * Note this doesn't require synchronization with the other CPUs, as > + * there are strictly no modifications of the current MTRR values. > */ > - set_mtrr(~0U, 0, 0, 0); > + mtrr_set_all(); > } While I agree with the change here, it doesn't go quite far enough. Originally I meant to ask that, with this (supposedly) sole use of ~0U gone, you please also drop the handling of that special case from set_mtrr(). But another similar call exist in mtrr_aps_sync_end(). Yet while that's "fine" for the boot case (watchdog being started only slightly later), it doesn't look to be for the S3 resume one: The watchdog is re-enabled quite a bit earlier there. I actually wonder whether mtrr_aps_sync_{begin,end}() wouldn't better themselves invoke watchdog_{dis,en}able(), thus also making the boot case explicitly safe, not just safe because of ordering. Jan