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Re: [PATCH v2] x86/vhpet: Fix sanitization of legacy IRQ route
- To: Tu Dinh <ngoc-tu.dinh@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 26 Nov 2025 07:53:47 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 26 Nov 2025 06:53:59 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 25.11.2025 17:52, 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")
> Signed-off-by: Tu Dinh <ngoc-tu.dinh@xxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
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