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Re: [PATCH v3 01/10] xen/version: Calculate xen_capabilities_info once at boot
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 16 Aug 2023 08:48:08 +0200
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Julien Grall <julien@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>, Henry Wang <Henry.Wang@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>
- Delivery-date: Wed, 16 Aug 2023 06:48:24 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 16.08.2023 01:03, Stefano Stabellini wrote:
> On Tue, 15 Aug 2023, Andrew Cooper wrote:
>> The arch_get_xen_caps() infrastructure is horribly inefficient for something
>> that is constant after features have been resolved on boot.
>>
>> Every instance used snprintf() to format constants into a string (which gets
>> shorter when %d gets resolved!), and which get double buffered on the stack.
>>
>> Switch to using string literals with the "3.0" inserted - these numbers
>> haven't changed in 18 years (The Xen 3.0 release was Dec 5th 2005).
>>
>> Use initcalls to format the data into xen_cap_info, which is deliberately not
>> of type xen_capabilities_info_t because a 1k array is a silly overhead for
>> storing a maximum of 77 chars (the x86 version) and isn't liable to need any
>> more space in the forseeable future.
>>
>> This speeds up the the XENVER_capabilities hypercall, but the purpose of the
>> change is to allow us to introduce a better XENVER_* API that doesn't force
>> the use of a 1k buffer on the stack.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>
> Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
albeit I still think your original concern regarding ...
>> @@ -537,7 +538,7 @@ long do_xen_version(int cmd,
>> XEN_GUEST_HANDLE_PARAM(void) arg)
>>
>> memset(info, 0, sizeof(info));
>> if ( !deny )
>> - arch_get_xen_caps(&info);
>> + safe_strcpy(info, xen_cap_info);
>>
>> if ( copy_to_guest(arg, info, ARRAY_SIZE(info)) )
>> return -EFAULT;
... the unhelpful use of a stack variable here could do with addressing.
But of course that can equally be done in a subsequent patch.
Jan
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