Re: [PATCH v1 19/27] xen/riscv: emulate guest writes to virtual APLIC MMIO

[Jan Beulich <jbeulich@suse.com>]

On 14.04.2026 18:04, Oleksii Kurochko wrote:
> On 4/2/26 4:18 PM, Jan Beulich wrote:
>> On 10.03.2026 18:08, Oleksii Kurochko wrote:
>>> @@ -127,6 +137,164 @@ int vaplic_map_device_irqs_to_domain(struct domain *d,
>>>       return 0;
>>>   }
>>>   
>>> +static void vaplic_dm_update_target(const unsigned long hart_id, uint32_t *iprio)
>>> +{
>>> +    *iprio &= APLIC_TARGET_IPRIO_MASK;
>>> +    *iprio |= (hart_id << APLIC_TARGET_HART_IDX_SHIFT);
>>> +}
>>> +
>>> +static void vaplic_update_target(const struct imsic_config *imsic,
>>> +                                 const int guest_id,
>>> +                                 const unsigned long hart_id, uint32_t *value)
>>> +{
>>> +    unsigned long group_index;
>>> +    unsigned int hhxw = imsic->group_index_bits;
>>> +    unsigned int lhxw = imsic->hart_index_bits;
>>> +    unsigned int hhxs = imsic->group_index_shift - IMSIC_MMIO_PAGE_SHIFT * 2;
>>> +    unsigned long base_ppn = imsic->msi[hart_id].base_addr >> IMSIC_MMIO_PAGE_SHIFT;
>>> +
>>> +    group_index = (base_ppn >> (hhxs + 12)) & (BIT(hhxw, UL) - 1);
>>
>> And there's no constant available to make this literal 12 more descriptive?
> 
> As it was used in aplic_set_irq_affinity() - IMSIC_MMIO_PAGE_SHIFT could 
> be used here.
> 
>>
>>> +    *value &= APLIC_TARGET_EIID_MASK;
>>> +    *value |= guest_id << APLIC_TARGET_GUEST_IDX_SHIFT;
>>> +    *value |= hart_id << APLIC_TARGET_HART_IDX_SHIFT;
>>> +    *value |= group_index << (lhxw + APLIC_TARGET_HART_IDX_SHIFT) ;
>>> +}
>>
>> Both functions returning void right now, why would they need to return their
>> result via indirection?
> 
> No specific reason. Do you think it would be better just to return value 
> instead? I am okay to rework that.

Rule of thumb is: Use return value in preference to indirection when the
returned value isn't needed for some other purpose.

>>> +#define CALC_REG_VALUE(base) \
>>> +{ \
>>> +    uint32_t index; \
>>> +    uint32_t tmp_val; \
>>
>> Combine these two, or have the variables have initializers?
>>
>>> +    index = regval_to_irqn(offset - base); \
>>
>> There's no "offset" declared or passed into here, nor ...
>>
>>> +    tmp_val = APLIC_REG_GET(priv->regs, aplic_addr) & ~auth_irq_bmp[index]; \
>>
>> ... "priv", nor ...
>>
>>> +    value &= auth_irq_bmp[index]; \
>>> +    value |= tmp_val; \
>>
>> ... "value". It may remain like this, but then it wants putting inside the
>> sole function that uses it, and be #undef-ed at the end of the function.
>>
>>> +}
>>
>> Please wrap in do/while(0), for use sites to be required to have semicolons
>> (and hence look like normal statements). Or make it a statement expression
>> properly returning the calculated value.
> 
> I will put the following inside the function + undef at the end:
> 
> #define CALC_REG_VALUE(base) do {                               \ 
>                                    \

Nit: Why this extra line?

>>> +static int cf_check vaplic_emulate_store(const struct vcpu *vcpu,
>>> +                                         unsigned long addr, uint32_t value)
>>> +{
>>> +    struct vaplic *vaplic = to_vaplic(vcpu->domain->arch.vintc);
>>> +    struct aplic_priv *priv = vaplic->base.info->private;
>>> +    uint32_t offset = addr & APLIC_REG_OFFSET_MASK;
>>
>> See ./CODING_STYLE as to uses of fixed-width types.
>>
>>> +    unsigned long aplic_addr = addr - priv->paddr_start;
>>> +    const uint32_t *auth_irq_bmp = vcpu->domain->arch.vintc->private;
>>> +
>>> +    switch ( offset )
>>> +    {
>>> +    case APLIC_SETIP_BASE ... APLIC_SETIP_LAST:
>>
>> And (taking this just as example) any misaligned accesses falling in this range
>> are fine?
> 
> Do you mean something like 0x1C02 instead of 0x1C00 or 0x1C04?

Yes.

>>> +        /*
>>> +         * As sourcecfg register starts from 1:
>>> +         *   0x0000 domaincfg
>>> +         *   0x0004 sourcecfg[1]
>>> +         *   0x0008 sourcecfg[2]
>>> +         *    ...
>>> +         *   0x0FFC sourcecfg[1023]
>>> +         * It is necessary to calculate an interrupt number by substracting
>>
>> Nit: subtracting
>>
>>> +         * of APLIC_DOMAINCFG instead of APLIC_SOURCECFG_BASE.
>>> +         */
>>> +        if ( !AUTH_IRQ_BIT(regval_to_irqn(offset - APLIC_DOMAINCFG)) )
>>> +            /* interrupt not enabled, ignore it */
>>
>> Throughout the series: Please adhere to ./CODING_STYLE.
>>
>>> +            return 0;
>>> +
>>> +        break;
>>
>> And any value is okay to write?
> 
> No, it should be in a range 
> [APLIC_SOURCECFG_SM_INACTIVE,APLIC_SOURCECFG_SM_LEVEL_LOW].
> 
> I will add the check before break:
>          if ( value > APLIC_SOURCECFG_SM_LEVEL_LOW )
>          {
>              gdprintk(XENLOG_WARNING,
>                       "value(%u) is incorrect for sourcecfg register\n", 
> value);
>              value = APLIC_SOURCECFG_SM_INACTIVE;
>          }

And why would writing APLIC_SOURCECFG_SM_INACTIVE be any better, when
that's not what the guest wanted? Simply ignore such writes, unless the
spec mandates specific behavior for out-of-range avlues?

>>> +    case APLIC_TARGET_BASE ... APLIC_TARGET_LAST:
>>> +        struct vcpu *target_vcpu = NULL;
>>> +
>>> +        /*
>>> +         * Look at vaplic_emulate_load() for explanation why
>>> +         * APLIC_GENMSI is substracted.
>>> +         */
>>
>> There's no vaplic_emulate_load() - how can I go look there?
> 
> It is introduced in the next patch.

As before - it should be possible to review patch series strictly
sequentially. Further, what if this patch gets committed, and the other
gets delayed by several months?

>>> +        if ( !AUTH_IRQ_BIT(regval_to_irqn(offset - APLIC_GENMSI)) )
>>> +            /* interrupt not enabled, ignore it */
>>> +            return 0;
>>> +
>>> +        for ( int i = 0; i < vcpu->domain->max_vcpus; i++ )
>>
>> unsigned int
>>
>>> +        {
>>> +            struct vcpu *v = vcpu->domain->vcpu[i];
>>> +
>>> +            if ( v->vcpu_id == (value >> APLIC_TARGET_HART_IDX_SHIFT) )
>>> +            {
>>> +                target_vcpu = v;
>>> +                break;
>>> +            }
>>> +        }
>>> +
>>> +        ASSERT(target_vcpu);
>>
>> What guarantees the pointer to be non-NULL? The incoming value can be
>> arbitrary, afaict.
> 
> I didn't understand your point. It is just checking that target_vcpu has 
> been found. If after for() loop the value of target_vcpu is still NULL 
> then something wrong in Xen.

If that's true, then the assertion is fine to have. I can't help the
impression though that a guest could pick a value such that you can't
possibly find the target vCPU. Asserting on guest controlled input is
not okay, as was said several times before.

>>> +        if ( !(vaplic->regs.domaincfg & APLIC_DOMAINCFG_DM) )
>>> +        {
>>> +            vaplic_dm_update_target(cpuid_to_hartid(target_vcpu->processor),
>>> +                                    &value);
>>> +        }
>>> +        else
>>> +            vaplic_update_target(priv->imsic_cfg,
>>> +                                 vcpu_guest_file_id(target_vcpu),
>>> +                                 cpuid_to_hartid(target_vcpu->processor),
>>> +                                 &value);
>>
>> I'm struggling with the naming here: When DM is clear, a function with "dm"
>> in the name is called.
> 
> it means direct (delivery) mode. Maybe it is better to put dm at the end 
> of the function name? Or it is just better to change it to something else?

Without a better understanding of what is wanted, all I can say is that
calling something with "dm" in its name when the condition says it's not
"dm" is confusing.

>>> +    default:
>>> +        panic("%s: unsupported register offset: %#x\n", __func__, offset);
>>
>> Crashing the host for the guest doing something odd? It's odd that the function
>> only ever returns 0 anyway - it could simply return an error here (if the
>> itention is to not ignore such writes).
> 
> But maybe it is a legal offset and we really want to support it?

Still not a reason to crash the entire host?

> Even if I will return just error then a caller site will want to do 
> something with this error -> for example, kill domain or panic() again. 
> Maybe panic is to much and just domain should be crashed here:
> 
> default:
>      gdprintk(XENLOG_WARNING,
>               "Unhandled APLIC write at offset %#x (value %#x)\n",
>               offset, value);
>      domain_crash(vcpu->domain);
>      return 0;
> ?

This would already be better. You shouldn't use gdprintk() with
domain_crash() though. Please take a look at domain_crash()'s
definition - you'll then see what to do, I suppose.

Jan