Re: [PATCH 1/2] xen/arm: Improve handling of nr_spis

[Bertrand Marquis <Bertrand.Marquis@xxxxxxx>]

> On 11 Mar 2025, at 10:59, Orzel, Michal <Michal.Orzel@xxxxxxx> wrote:
> 
> 
> 
> On 11/03/2025 10:30, Bertrand Marquis wrote:
>> 
>> 
>> Hi Michal,
>> 
>>> On 11 Mar 2025, at 10:04, Michal Orzel <michal.orzel@xxxxxxx> wrote:
>>> 
>>> At the moment, we print a warning about max number of IRQs supported by
>>> GIC bigger than vGIC only for hardware domain. This check is not hwdom
>>> special, and should be made common. Also, in case of user not specifying
>>> nr_spis for dom0less domUs, we should take into account max number of
>>> IRQs supported by vGIC if it's smaller than for GIC.
>>> 
>>> Introduce VGIC_MAX_IRQS macro and use it instead of hardcoded 992 value.
>>> Fix calculation of nr_spis for dom0less domUs and make the GIC/vGIC max
>>> IRQs comparison common.
>>> 
>>> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
>>> ---
>>> xen/arch/arm/dom0less-build.c   | 2 +-
>>> xen/arch/arm/domain_build.c     | 9 ++-------
>>> xen/arch/arm/gic.c              | 3 +++
>>> xen/arch/arm/include/asm/vgic.h | 3 +++
>>> 4 files changed, 9 insertions(+), 8 deletions(-)
>>> 
>>> diff --git a/xen/arch/arm/dom0less-build.c b/xen/arch/arm/dom0less-build.c
>>> index 31f31c38da3f..9a84fee94119 100644
>>> --- a/xen/arch/arm/dom0less-build.c
>>> +++ b/xen/arch/arm/dom0less-build.c
>>> @@ -1018,7 +1018,7 @@ void __init create_domUs(void)
>>>        {
>>>            int vpl011_virq = GUEST_VPL011_SPI;
>>> 
>>> -            d_cfg.arch.nr_spis = gic_number_lines() - 32;
>>> +            d_cfg.arch.nr_spis = min(gic_number_lines(), VGIC_MAX_IRQS) - 
>>> 32;
>> 
>> I would suggest to introduce a static inline gic_nr_spis in a gic header ...
> Why GIC and not vGIC? This is domain's nr_spis, so vGIC.

yes vGIC sorry.

> But then, why static inline if the value does not change and is domain 
> agnostic?
> I struggle to find a good name for this macro. Maybe (in vgic.h):
> #define vgic_def_nr_spis (min(gic_number_lines(), VGIC_MAX_IRQS) - 32)
> to denote default nr_spis if not set by the user?

Yes that would work. My point is to prevent to have 2 definitions in 2 different
source file and a risk to forget to update one and not the other (let say if 
some
day we change 32 in 64).

> 
>> 
>>> 
>>>            /*
>>>             * The VPL011 virq is GUEST_VPL011_SPI, unless direct-map is
>>> diff --git a/xen/arch/arm/domain_build.c b/xen/arch/arm/domain_build.c
>>> index 7cc141ef75e9..b99c4e3a69bf 100644
>>> --- a/xen/arch/arm/domain_build.c
>>> +++ b/xen/arch/arm/domain_build.c
>>> @@ -2371,13 +2371,8 @@ void __init create_dom0(void)
>>> 
>>>    /* The vGIC for DOM0 is exactly emulating the hardware GIC */
>>>    dom0_cfg.arch.gic_version = XEN_DOMCTL_CONFIG_GIC_NATIVE;
>>> -    /*
>>> -     * Xen vGIC supports a maximum of 992 interrupt lines.
>>> -     * 32 are substracted to cover local IRQs.
>>> -     */
>>> -    dom0_cfg.arch.nr_spis = min(gic_number_lines(), (unsigned int) 992) - 
>>> 32;
>>> -    if ( gic_number_lines() > 992 )
>>> -        printk(XENLOG_WARNING "Maximum number of vGIC IRQs exceeded.\n");
>>> +    /* 32 are substracted to cover local IRQs */
>>> +    dom0_cfg.arch.nr_spis = min(gic_number_lines(), VGIC_MAX_IRQS) - 32;
>> 
>> and reuse it here to make sure the value used is always the same.
>> 
>>>    dom0_cfg.arch.tee_type = tee_get_type();
>>>    dom0_cfg.max_vcpus = dom0_max_vcpus();
>>> 
>>> diff --git a/xen/arch/arm/gic.c b/xen/arch/arm/gic.c
>>> index acf61a4de373..e80fe0ca2421 100644
>>> --- a/xen/arch/arm/gic.c
>>> +++ b/xen/arch/arm/gic.c
>>> @@ -251,6 +251,9 @@ void __init gic_init(void)
>>>        panic("Failed to initialize the GIC drivers\n");
>>>    /* Clear LR mask for cpu0 */
>>>    clear_cpu_lr_mask();
>>> +
>>> +    if ( gic_number_lines() > VGIC_MAX_IRQS )
>>> +        printk(XENLOG_WARNING "Maximum number of vGIC IRQs exceeded\n");
>> 
>> I am a bit unsure with this one.
>> If this is the case, it means any gicv2 or gicv3 init detected an impossible 
>> value and
>> any usage of gic_number_lines would be using an impossible value.
> Why impossible? GIC can support up to 1020 IRQs. Our vGIC can support up to 
> 992
> IRQs.

Maybe unsupported is a better wording, my point is that it could lead to non 
working system
if say something uses irq 1000.

> 
>> 
>> Shouldn't this somehow result in a panic as the gic detection was wrong ?
>> Do you think we can continue to work safely if this value is over 
>> VGIC_MAX_IRQS.
>> There are other places using gic_number_lines like in irq.c.
> I can add a panic, sure. This would be a change in behavior because we used 
> this
> check for hwdom unconditionally. I'd need others opinion for this one.

ok.

Cheers
Bertrand

> 
> ~Michal