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Thu, 05 Feb 2026 03:33:52 -0800 (PST) Message-ID: <1e2df446-b116-4861-a396-b87895a96c67@suse.com> Date: Thu, 5 Feb 2026 12:33:49 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Content-Language: en-US To: "xen-devel@lists.xenproject.org" Cc: Julien Grall , Stefano Stabellini , Volodymyr Babchuk , Bertrand Marquis , Michal Orzel , Juergen Gross , Dario Faggioli , George Dunlap From: Jan Beulich Subject: [PATCH] sched: move vCPU exec state barriers 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: 1770291263610154100 Content-Type: text/plain; charset="utf-8" The barrier in vcpu_context_saved() is specifically about the clearing of ->is_running. It isn't needed when we don't clear the flag. Furthermore, one side of the barrier being in common code, the other would better be, too. This way, all architectures are covered (beyond any specific needs they may have). Signed-off-by: Jan Beulich Reviewed-by: Juergen Gross --- a/xen/arch/arm/domain.c +++ b/xen/arch/arm/domain.c @@ -333,17 +333,7 @@ void sync_local_execstate(void) =20 void sync_vcpu_execstate(struct vcpu *v) { - /* - * We don't support lazy switching. - * - * However the context may have been saved from a remote pCPU so we - * need a barrier to ensure it is observed before continuing. - * - * Per vcpu_context_saved(), the context can be observed when - * v->is_running is false (the caller should check it before calling - * this function). - */ - smp_rmb(); + /* Nothing to do -- no lazy switching */ } =20 #define NEXT_ARG(fmt, args) = \ --- a/xen/common/sched/core.c +++ b/xen/common/sched/core.c @@ -956,6 +956,9 @@ void vcpu_sleep_sync(struct vcpu *v) while ( !vcpu_runnable(v) && v->is_running ) cpu_relax(); =20 + /* Sync state /after/ observing the running flag clear. */ + smp_rmb(); + sync_vcpu_execstate(v); } =20 @@ -2308,11 +2311,13 @@ static struct sched_unit *do_schedule(st =20 static void vcpu_context_saved(struct vcpu *vprev, struct vcpu *vnext) { - /* Clear running flag /after/ writing context to memory. */ - smp_wmb(); - if ( vprev !=3D vnext ) + { + /* Clear running flag /after/ writing context to memory. */ + smp_wmb(); + vprev->is_running =3D false; + } } =20 static void unit_context_saved(struct sched_resource *sr)