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Tue, 05 Dec 2023 07:16:26 -0800 (PST) Message-ID: Date: Tue, 5 Dec 2023 16:16:25 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/5] xen/livepatch: register livepatch regions when loaded Content-Language: en-US To: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= Cc: Konrad Rzeszutek Wilk , Ross Lagerwall , Andrew Cooper , George Dunlap , Julien Grall , Stefano Stabellini , Wei Liu , xen-devel@lists.xenproject.org References: <20231130142944.46322-1-roger.pau@citrix.com> <20231130142944.46322-2-roger.pau@citrix.com> <4bfb71ef-0443-40dd-a854-349db42a7a30@suse.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: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 05.12.2023 16:04, Roger Pau Monné wrote: > On Tue, Dec 05, 2023 at 02:47:56PM +0100, Jan Beulich wrote: >> On 30.11.2023 15:29, Roger Pau Monne wrote: >>> diff --git a/xen/common/virtual_region.c b/xen/common/virtual_region.c >>> index 5f89703f513b..b444253848cf 100644 >>> --- a/xen/common/virtual_region.c >>> +++ b/xen/common/virtual_region.c >>> @@ -23,14 +23,8 @@ static struct virtual_region core_init __initdata = { >>> }; >>> >>> /* >>> - * RCU locking. Additions are done either at startup (when there is only >>> - * one CPU) or when all CPUs are running without IRQs. >>> - * >>> - * Deletions are bit tricky. We do it when Live Patch (all CPUs running >>> - * without IRQs) or during bootup (when clearing the init). >>> - * >>> - * Hence we use list_del_rcu (which sports an memory fence) and a spinlock >>> - * on deletion. >>> + * RCU locking. Modifications to the list must be done in exclusive mode, and >>> + * hence need to hold the spinlock. >>> * >>> * All readers of virtual_region_list MUST use list_for_each_entry_rcu. >>> */ >>> @@ -58,38 +52,28 @@ const struct virtual_region *find_text_region(unsigned long addr) >>> >>> void register_virtual_region(struct virtual_region *r) >>> { >>> - ASSERT(!local_irq_is_enabled()); >>> + unsigned long flags; >>> >>> + spin_lock_irqsave(&virtual_region_lock, flags); >>> list_add_tail_rcu(&r->list, &virtual_region_list); >>> + spin_unlock_irqrestore(&virtual_region_lock, flags); >>> } >>> >>> static void remove_virtual_region(struct virtual_region *r) >>> { >>> - unsigned long flags; >>> + unsigned long flags; >> >> Nit: Stray blank added? > > Oh, my bad. > >>> - spin_lock_irqsave(&virtual_region_lock, flags); >>> - list_del_rcu(&r->list); >>> - spin_unlock_irqrestore(&virtual_region_lock, flags); >>> - /* >>> - * We do not need to invoke call_rcu. >>> - * >>> - * This is due to the fact that on the deletion we have made sure >>> - * to use spinlocks (to guard against somebody else calling >>> - * unregister_virtual_region) and list_deletion spiced with >>> - * memory barrier. >>> - * >>> - * That protects us from corrupting the list as the readers all >>> - * use list_for_each_entry_rcu which is safe against concurrent >>> - * deletions. >>> - */ >>> + spin_lock_irqsave(&virtual_region_lock, flags); >>> + list_del_rcu(&r->list); >>> + spin_unlock_irqrestore(&virtual_region_lock, flags); >>> } >>> >>> void unregister_virtual_region(struct virtual_region *r) >>> { >>> - /* Expected to be called from Live Patch - which has IRQs disabled. */ >>> - ASSERT(!local_irq_is_enabled()); >>> - >>> remove_virtual_region(r); >>> + >>> + /* Assert that no CPU might be using the removed region. */ >>> + rcu_barrier(); >>> } >> >> rcu_barrier() is a relatively heavy operation aiui. Seeing ... >> >>> #if defined(CONFIG_LIVEPATCH) && defined(CONFIG_X86) >> >> ... this I'd like to ask to consider hiding {,un}register_virtual_region() >> in "#ifdef CONFIG_LIVEPATCH" as well, to make clear these aren't supposed >> to be used for other purpose. Would at the same time address two Misra >> violations, I think (both functions having no callers when !LIVEPATCH). > > That's fine, I can do it this same patch unless you prefer such > adjustment to be in a separate change. Since the change itself constitutes at least part of the reason for the adjustment, this would be fine with me. (A separate change, if preferred by others, would still be fine, too.) Jan