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Re: [PATCH Xen 4.20] x86/apic: Fix asm() constraints in TMICT calculation
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Tue, 21 Apr 2026 11:22:42 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Tue, 21 Apr 2026 10:22:53 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 21/04/2026 7:39 am, Jan Beulich wrote:
> On 20.04.2026 19:46, Andrew Cooper wrote:
>> The encoded MUL is 64 bits, so writes %rdx too. At a minimum, this needs
>> expressing as a clobber.
> I'm embarrassed of missing this.
>
>> Also fix a logical disconnect between 'overflow' being the carry flag not the
>> overflow flag. CF and OF are always the same for MUL instructions, so use
>> the
>> flag which matches the variable name.
> I don't mind this too much, but the use of CF was deliberate: Imo OF is
> relevant to signed arithmetic only, whereas CF is the flag to use with
> unsigned operations.
>
>> Fixes: d5c70a51bfbe ("x86/APIC: handle overflow in TMICT calculation")
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
Thanks. I could rename the variable to carry instead then? Either works.
>
>> I've kept this form because it produces best code generation for GCCs which
>> support flag outputs.
>>
>> An alternative would be to capture product_hi and check the nonzero-ness, as
>> that's how OF/CF are produced in hardware, which would be better code
>> generation on very old GCCs.
> We could fit both, by further widening the use of ASM_FLAG_OUT().
>
>> --- a/xen/arch/x86/apic.c
>> +++ b/xen/arch/x86/apic.c
>> @@ -1317,9 +1317,10 @@ int reprogram_timer(s_time_t timeout)
>>
>> apic_tmict = UINT32_MAX;
>> asm ( "mul %[expire]\n\t"
>> - ASM_FLAG_OUT(, "setc %[cf]")
>> - : "=a" (product), [cf] ASM_FLAG_OUT("=@ccc", "=qm") (overflow)
>> - : "0" ((unsigned long)bus_scale), [expire] "r" (expire) );
>> + ASM_FLAG_OUT(, "seto %[of]")
>> + : "=a" (product), [of] ASM_FLAG_OUT("=@cco", "=qm") (overflow)
> Noticing only now - for the non-flag-output case this should be "=&a". With
>
>> + : "0" ((unsigned long)bus_scale), [expire] "r" (expire)
> ... %rax also being an input, there's no risk of the compiler using the
> register for the other input, but still. Would you mind making that adjustment
> as well, while at it?
Ok.
~Andrew
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