Re: [PART2 PATCH v4 07/11] iommu/amd: Introduce amd_iommu_update_ga()

Radim Krčmář
2016-07-14T13:41:04Z

2016-07-14 16:13+0700, Suravee Suthikulpanit:
> On 7/13/16 21:14, Radim Krčmář wrote:
>> 2016-07-13 08:20-0500, Suravee Suthikulpanit:
>> > diff --git a/drivers/iommu/amd_iommu.c b/drivers/iommu/amd_iommu.c
>> > @@ -4461,4 +4461,69 @@ int amd_iommu_create_irq_domain(struct amd_iommu *iommu)
>> > +int amd_iommu_update_ga(u32 vcpu_id, u32 cpu, u32 vm_id,
>> > + u64 base, bool is_run)
>>
>> |2016-07-13 15:49+0700, Suravee Suthikulpanit:
>> |> On 07/12/2016 01:59 AM, Radim Krčmář wrote:
>> |>> Not just in this function does the interface between svm and iommu split
>> |>> ga_tag into its two components (vcpu_id and ga_tag), but it seems that
>> |>> the combined value could always be used instead ...
>> |>> Is there an advantage to passing two values?
>> |>
>> |> Here, the amd_iommu_update_ga() takes the two separate value for input
>> |> parameters. Mainly the ga_tag (which is really the vm_id) and vcpu_id. This
>> |> allow IOMMU driver to decide how to encode the GATAG to be programmed into
>> |> the IRTE. Currently, the actual GATAG is a 16-bit value, <vm_id><vcpu_id>.
>> |> This keeps the interface independent from how we encode the GATAG.
>>
>> I was thinking about making the IOMMU unaware how SVM or other Linux
>> hypervisors use the ga_tag, i.e. passing the final u32 ga_tag.
>> For example 32 bit hypervisor doesn't need to use lookup, because any
>> pointer can used as the ga_tag directly.
>
> Ahh....... (w/ a big light bulb)
> I get your point now. Let's just have SVM (or other hypervisor) define what
> the tag should be and just pass-on the value to IOMMU. IOMMU can just simply
> use this w/o knowing what it is. Sorry, I'm slow :)

That is what I meant, but misunderstanding is a product of both
participants. I didn't write it clearly on the first try.

>> > + hash_for_each_possible(iommu->gatag_ir_hash, ir_data, hnode,
>> > + AMD_IOMMU_GATAG(vm_id, vcpu_id)) {
>> > + struct irte_ga *irte = (struct irte_ga *) ir_data->entry;
>>
>> |>> (The ga_tag check is missing here too.)
>> |>
>> |> Here, the intention is to update all interrupt remapping entries in the
>> |> bucket w/ the same GATAG (i.e. vm_id + vcpu_id), where GATAG =
>> |> AMD_IOMMU_GATAG(vm_id, vcpu_id).
>>
>> Which is why you need to check that
>> AMD_IOMMU_GATAG(vm_id, vcpu_id) == entry->fields_vapic.ga_tag
>>
>> The hashing function can map two different vm_id + vcpu_id to the same
>> bucket and hash_for_each_possible() would return both of them, but only
>> one belongs to the VCPU that we want to update.
>>
>> (And shouldn't there be only one match?)
>
> Actually, with your suggestion above, the hask key would be (vm_id &
> 0x3FFFFF << 8)| (vcpu_id & 0xFF). So, it should be unique for each vcpu of
> each vm, or am I still missing something?

[Reply in the followup mail.]

> Also, since we will not be passing the vmid and vcpuid as separate value,
> and just passing the (u32)ga_tag, we would not be able to do the check you
> suggested here.

There will be the u32 ga_tag argument, so you would still do
ga_tag == entry->fields_vapic.ga_tag

Because even if the ga_tag is unique for every vcpu, the hash table will
mix various vcpus into one bucket and you need to filter them.

>> > + update_irte_ga((struct irte_ga *)ir_data->ref,
>> > + ir_data->irq_2_irte.devid,
>> > + base, cpu, is_run);
>>
>> |>> (The lookup leading up to here is avoidable -- svm, the caller, has the
>> |>> ability to map ga_tag into irte/ir_data directly with a pointer.
>
> I'm not sure about this optimization to avoid look up.
>
> The struct amd_ir_data is part of the IOMMU driver, and the SVM knows
> nothing about it. I don't think it would be able to find out the pointer to
> amd_ir_data/irte.

Yeah, SVM would store it in a "void *" pointer, because it doesn't need
to know anything else, but you still need to retrieve it from IOMMU,
which could be done through vcpu_info argument to
amd_ir_set_vcpu_affinity().

(I am not sure if it doesn't breach isolation of IOMMU, so we might not
want to do it in any case ...)

> Also, with the current design, each ga_tag can be mapped to different irte
> since there could be multiple interrupts targeting a particular cpu. Here,
> we would want to update all of the IRTEs with the same ga_tag.

True, that design is good. SVM would need a list of pointers for each
vcpu to cope with it ...

>> |>> I'm not sure if the lookup is slow enough to pardon optimization, but
>> |>> it might make the code simpler as well.)
>> |>
>> |> I might have mislead you up to this point. Not sure if the assumption here
>> |> still hold with my explanation above. Sorry for confusion.
>>
>> SVM configures IOMMU with ga_tag, so IOMMU could return the pointer to
>> ir_data/irte that was just configured.
>
> Also, IIUC, you want to use the pointer to ir_data/irte as the ga_tag value.
> The issue would be ga_tag is a 32-bit value, and this would not work with
> 64-bit address.

I mean something slightly different. Instead of passing ga_tag into
amd_iommu_update_ga(), just pass void * of whatever IOMMU provided back
when SVM configured the interrupt. ga_tag will never come into play.

(The vcpu lookup from ga_tag is necessary, when processing the queue of
undelivered interrupts. ir_data lookup can be avoided.)