Re: [PATCH] x86/vhpet: Fix sanitization of legacy IRQ route

["Tu Dinh" <ngoc-tu.dinh@xxxxxxxxxx>]

On 24/11/2025 15:53, Jan Beulich wrote:
> On 24.11.2025 14:43, Tu Dinh wrote:
>> When setting a timer's config register, timer_sanitize_int_route will
>> always reset the IRQ route value to what's valid corresponding to the
>> !HPET_CFG_LEGACY case. This is applied even if the HPET is set to
>> HPET_CFG_LEGACY.
>>
>> When some operating systems (e.g. Windows) try to write to a timer
>> config, they will verify and rewrite the register if the values don't
>> match what they expect. This causes an unnecessary write to HPET_Tn_CFG.
>>
>> Note, the HPET specification states that for the Tn_INT_ROUTE_CNF field:
>>
>> "If the value is not supported by this prarticular timer, then the value
>> read back will not match what is written. [...] If the LegacyReplacement
>> Route bit is set, then Timers 0 and 1 will have a different routing, and
>> this bit field has no effect for those two timers."
>>
>> Therefore, Xen should not reset timer_int_route if legacy mode is
>> enabled, regardless of what's in there.
> 
> Fixes: ec40d3fe2147 ("x86/vhpet: check that the set interrupt route is valid")
> (I think)

Yes, thanks.

> 
>> Signed-off-by: Tu Dinh <ngoc-tu.dinh@xxxxxxxxxx>
>> ---
>>   xen/arch/x86/hvm/hpet.c | 11 ++++++++---
>>   1 file changed, 8 insertions(+), 3 deletions(-)
>>
>> --- a/xen/arch/x86/hvm/hpet.c
>> +++ b/xen/arch/x86/hvm/hpet.c
>> @@ -48,6 +48,8 @@
>>   #define timer_is_32bit(h, n)     (timer_config(h, n) & HPET_TN_32BIT)
>>   #define hpet_enabled(h)          ((h)->hpet.config & HPET_CFG_ENABLE)
>>   #define timer_level(h, n)        (timer_config(h, n) & HPET_TN_LEVEL)
>> +#define timer_is_legacy(h, n) \
>> +    (((n) <= 1) && ((h)->hpet.config & HPET_CFG_LEGACY))
>>   
>>   #define timer_int_route(h, n)    MASK_EXTR(timer_config(h, n), 
>> HPET_TN_ROUTE)
>>   
>> @@ -244,7 +246,7 @@ static void hpet_set_timer(HPETState *h, unsigned int tn,
>>            (timer_level(h, tn) && test_bit(tn, &h->hpet.isr)) )
>>           return;
>>   
>> -    if ( !timer_int_route_valid(h, tn) )
>> +    if ( !timer_is_legacy(h, tn) && !timer_int_route_valid(h, tn) )
> 
> Seeing this and the other use together with timer_int_route_valid(),
> wouldn't timer_int_route_valid() better itself invoke the new macro?

I thought about it, but I found that it was overloading the definition 
of timer_int_route_valid a little too much. Since timer_is_legacy() is 
being used by itself later anyway, I figured it'd be better to just 
separate the two.

> 
>> @@ -379,6 +381,9 @@ static int cf_check hpet_write(
>>           h->hpet.config = hpet_fixup_reg(new_val, old_val,
>>                                           HPET_CFG_ENABLE | HPET_CFG_LEGACY);
>>   
>> +        for ( i = 0; i < HPET_TIMER_NUM; i++ )
>> +            timer_sanitize_int_route(h, i);
> 
> Strictly speaking this is needed only when HPET_CFG_LEGACY went from
> 1 to 0. Plus it's needed only on the first 2 channels, as it's only
> there where timer_sanitize_int_route() changes behavior. I'm not going
> to insist to limit it like this, but if you don't, then I'd like to ask
> for a comment here clarifying that excess work is done for simplicity's
> sake.

Agreed, I can limit it to i <= 1.

I'll prepare a v2.

> 
> Jan
> 



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