Subject: Re: [PATCH kernel 8/9] KVM: PPC: Add in-kernel handling for VFIO

From: Alexey Kardashevskiy <[email-masked]>
Date: Fri, 11 Mar 2016 13:15:20 +1100

On 03/10/2016 04:18 PM, David Gibson wrote:
> On Wed, Mar 09, 2016 at 07:46:47PM +1100, Alexey Kardashevskiy wrote:
>> On 03/08/2016 10:08 PM, David Gibson wrote:
>>> On Mon, Mar 07, 2016 at 02:41:16PM +1100, Alexey Kardashevskiy wrote:
>>>> This allows the host kernel to handle H_PUT_TCE, H_PUT_TCE_INDIRECT
>>>> and H_STUFF_TCE requests targeted an IOMMU TCE table used for VFIO
>>>> without passing them to user space which saves time on switching
>>>> to user space and back.
>>>>
>>>> Both real and virtual modes are supported. The kernel tries to
>>>> handle a TCE request in the real mode, if fails it passes the request
>>>> to the virtual mode to complete the operation. If it a virtual mode
>>>> handler fails, the request is passed to user space; this is not expected
>>>> to happen ever though.
>>>
>>> Well... not expect to happen with a qemu which uses this.  Presumably
>>> it will fall back to userspace routinely if you have an old qemu that
>>> doesn't add the liobn mappings.
>>
>>
>> Ah. Ok, thanks, I'll add this to the commit log.
>
> Ok.
>
>>>> The first user of this is VFIO on POWER. Trampolines to the VFIO external
>>>> user API functions are required for this patch.
>>>
>>> I'm not sure what you mean by "trampoline" here.
>>
>> For example, look at kvm_vfio_group_get_external_user. It calls
>> symbol_get(vfio_group_get_external_user) and then calls a function via the
>> returned pointer.
>>
>> Is there a better word for this?
>
> Uh.. probably although I don't immediately know what.  "Trampoline"
> usually refers to code on the stack used for bouncing places, which
> isn't what this resembles.

"Dynamic wrapper"?

>>>> This uses a VFIO KVM device to associate a logical bus number (LIOBN)
>>>> with an VFIO IOMMU group fd and enable in-kernel handling of map/unmap
>>>> requests.
>>>
>>> Group fd?  Or container fd?  The group fd wouldn't make a lot of
>>> sense.
>>
>>
>> Group. KVM has no idea about containers.
>
> That's not going to fly.  Having a liobn registered against just one
> group in a container makes no sense at all.  Conceptually, if not
> physically, the container shares a single set of TCE tables.  If
> handling that means teaching KVM the concept of containers, then so be
> it.
>
> Btw, I'm not sure yet if extending the existing vfio kvm device to
> make the vfio<->kvm linkages makes sense.  I think the reason some x86
> machines need that is quite different from how we're using it for
> Power.  I haven't got a clear enough picture yet to be sure either
> way.
>
> The other option that would seem likely to me would be a "bind VFIO
> container" ioctl() on the fd associated with a kernel accelerated TCE table.

Oh, I just noticed this response. I need to digest it. Looks like this is 
going to take other 2 years to upstream...

>>>> To make use of the feature, the user space has to create a guest view
>>>> of the TCE table via KVM_CAP_SPAPR_TCE/KVM_CAP_SPAPR_TCE_64 and
>>>> then associate a LIOBN with this table via VFIO KVM device,
>>>> a KVM_DEV_VFIO_GROUP_SET_SPAPR_TCE_LIOBN property (which is added in
>>>> the next patch).
>>>>
>>>> Tests show that this patch increases transmission speed from 220MB/s
>>>> to 750..1020MB/s on 10Gb network (Chelsea CXGB3 10Gb ethernet card).
>>>
>>> Is that with or without DDW (i.e. with or without a 64-bit DMA window)?
>>
>>
>> Without DDW, I should have mentioned this. The patch is from the times when
>> there was no DDW :(
>
> Ok.
>
>>>> Signed-off-by: Alexey Kardashevskiy <[email-masked]>
>>>> ---
>>>>   arch/powerpc/kvm/book3s_64_vio.c    | 184 +++++++++++++++++++++++++++++++++++
>>>>   arch/powerpc/kvm/book3s_64_vio_hv.c | 186 ++++++++++++++++++++++++++++++++++++
>>>>   2 files changed, 370 insertions(+)
>>>>
>>>> diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
>>>> index 7965fc7..9417d12 100644
>>>> --- a/arch/powerpc/kvm/book3s_64_vio.c
>>>> +++ b/arch/powerpc/kvm/book3s_64_vio.c
>>>> @@ -33,6 +33,7 @@
>>>>   #include <asm/kvm_ppc.h>
>>>>   #include <asm/kvm_book3s.h>
>>>>   #include <asm/mmu-hash64.h>
>>>> +#include <asm/mmu_context.h>
>>>>   #include <asm/hvcall.h>
>>>>   #include <asm/synch.h>
>>>>   #include <asm/ppc-opcode.h>
>>>> @@ -317,11 +318,161 @@ fail:
>>>>   	return ret;
>>>>   }
>>>>
>>>> +static long kvmppc_tce_iommu_mapped_dec(struct iommu_table *tbl,
>>>> +		unsigned long entry)
>>>> +{
>>>> +	struct mm_iommu_table_group_mem_t *mem = NULL;
>>>> +	const unsigned long pgsize = 1ULL << tbl->it_page_shift;
>>>> +	unsigned long *pua = IOMMU_TABLE_USERSPACE_ENTRY(tbl, entry);
>>>> +
>>>> +	if (!pua)
>>>> +		return H_HARDWARE;
>>>> +
>>>> +	mem = mm_iommu_lookup(*pua, pgsize);
>>>> +	if (!mem)
>>>> +		return H_HARDWARE;
>>>> +
>>>> +	mm_iommu_mapped_dec(mem);
>>>> +
>>>> +	*pua = 0;
>>>> +
>>>> +	return H_SUCCESS;
>>>> +}
>>>> +
>>>> +static long kvmppc_tce_iommu_unmap(struct iommu_table *tbl,
>>>> +		unsigned long entry)
>>>> +{
>>>> +	enum dma_data_direction dir = DMA_NONE;
>>>> +	unsigned long hpa = 0;
>>>> +
>>>> +	if (iommu_tce_xchg(tbl, entry, &hpa, &dir))
>>>> +		return H_HARDWARE;
>>>> +
>>>> +	if (dir == DMA_NONE)
>>>> +		return H_SUCCESS;
>>>> +
>>>> +	return kvmppc_tce_iommu_mapped_dec(tbl, entry);
>>>> +}
>>>> +
>>>> +long kvmppc_tce_iommu_map(struct kvm *kvm, struct iommu_table *tbl,
>>>> +		unsigned long entry, unsigned long gpa,
>>>> +		enum dma_data_direction dir)
>>>> +{
>>>> +	long ret;
>>>> +	unsigned long hpa, ua, *pua = IOMMU_TABLE_USERSPACE_ENTRY(tbl, entry);
>>>> +	struct mm_iommu_table_group_mem_t *mem;
>>>> +
>>>> +	if (!pua)
>>>> +		return H_HARDWARE;
>>>
>>> H_HARDWARE?  Or H_PARAMETER?  This essentially means the guest has
>>> supplied a bad physical address, doesn't it?
>>
>> Well, may be. I'll change. If it not H_TOO_HARD, it does not make any
>> difference after all :)
>>
>>
>>
>>>> +	if (kvmppc_gpa_to_ua(kvm, gpa, &ua, NULL))
>>>> +		return H_HARDWARE;
>>>> +
>>>> +	mem = mm_iommu_lookup(ua, 1ULL << tbl->it_page_shift);
>>>> +	if (!mem)
>>>> +		return H_HARDWARE;
>>>> +
>>>> +	if (mm_iommu_ua_to_hpa(mem, ua, &hpa))
>>>> +		return H_HARDWARE;
>>>> +
>>>> +	if (mm_iommu_mapped_inc(mem))
>>>> +		return H_HARDWARE;
>>>> +
>>>> +	ret = iommu_tce_xchg(tbl, entry, &hpa, &dir);
>>>> +	if (ret) {
>>>> +		mm_iommu_mapped_dec(mem);
>>>> +		return H_TOO_HARD;
>>>> +	}
>>>> +
>>>> +	if (dir != DMA_NONE)
>>>> +		kvmppc_tce_iommu_mapped_dec(tbl, entry);
>>>> +
>>>> +	*pua = ua;
>>>
>>> IIUC this means you have a copy of the UA for every group attached to
>>> the TCE table, but they'll all be the same. Any way to avoid that
>>> duplication?
>>
>> It is for every container, not a group. On P8, I allow multiple groups to go
>> to the same container, that means that a container has one or two
>> iommu_table, and each iommu_table has this "ua" list but since tables are
>> different (window size, page size, content), these "ua" arrays are also
>> different.
>
> Erm.. but h_put_tce iterates h_put_tce_iommu through all the groups
> attached to the stt, and each one seems to update pua.
>
> Or is that what the if (kg->tbl == tbltmp) continue; is supposed to
> avoid?  In which case what ensures that the stt->groups list is
> ordered by tbl pointer?

Nothing. In the normal case (POWER8 IODA2) all groups on the same liobn 
have the same iommu_table, so the first group's one gets updated, other do 
not but it is ok as they use the same table.

In a bad case (POWER7 IODA1, multiple containers per LIOBN) the same @ua 
can be updated more than once. Well, not a huge loss.

-- 
Alexey