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Re: [PATCH] x86/APIC: handle overflow in TMICT calculation
- To: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
- Date: Thu, 9 Apr 2026 21:29:10 -0400
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>
- Delivery-date: Fri, 10 Apr 2026 01:29:36 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 4/9/26 05:21, Jan Beulich wrote:
> With an expiry value on the order of 20 hours, and with a bus scale value
If I did my math correctly, I believe 10 hours is sufficient to trigger signed
multiplication overflow. 20 hours would result in unsigned multiplication
overflow.
> of 256k (as supplied by qemu), the (signed) multiplication will be UB. As
> we've checked that the value is positive, we mean unsigned multiplication
> anyway. Yet let's play safe against even larger expiry and bus scale
> values, leveraging the compiler builtin that there is for this purpose.
>
> While there also drop the stray cast from the actual TMICT write.
>
> Fixes: 9062553a0dc1 ("added time and accurate timer support")
> Fixes: b95beb185810 ("x86: Clean up APIC local timer handling")
> Reported-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Tested-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
Thanks!
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