Subject: Re: [PATCH 1/4] KVM: PPC: Add support for multiple-TCE hcalls

From: Alexander Graf <[email-masked]>
Date: Mon, 17 Jun 2013 12:46:30 +0200

On 06/17/2013 10:51 AM, Alexey Kardashevskiy wrote:
> On 06/17/2013 06:40 PM, Alexander Graf wrote:
>> On 17.06.2013, at 10:34, Alexey Kardashevskiy wrote:
>>
>>> On 06/17/2013 06:02 PM, Alexander Graf wrote:
>>>> On 17.06.2013, at 09:55, Alexey Kardashevskiy wrote:
>>>>
>>>>> On 06/17/2013 08:06 AM, Alexander Graf wrote:
>>>>>> On 05.06.2013, at 08:11, Alexey Kardashevskiy wrote:
>>>>>>
>>>>>>> This adds real mode handlers for the H_PUT_TCE_INDIRECT and
>>>>>>> H_STUFF_TCE hypercalls for QEMU emulated devices such as
>>>>>>> IBMVIO devices or emulated PCI.  These calls allow adding
>>>>>>> multiple entries (up to 512) into the TCE table in one call
>>>>>>> which saves time on transition to/from real mode.
>>>>>>>
>>>>>>> This adds a tce_tmp cache to kvm_vcpu_arch to save valid TCEs
>>>>>>> (copied from user and verified) before writing the whole list
>>>>>>> into the TCE table. This cache will be utilized more in the
>>>>>>> upcoming VFIO/IOMMU support to continue TCE list processing in
>>>>>>> the virtual mode in the case if the real mode handler failed
>>>>>>> for some reason.
>>>>>>>
>>>>>>> This adds a guest physical to host real address converter and
>>>>>>> calls the existing H_PUT_TCE handler. The converting function
>>>>>>> is going to be fully utilized by upcoming VFIO supporting
>>>>>>> patches.
>>>>>>>
>>>>>>> This also implements the KVM_CAP_PPC_MULTITCE capability, so
>>>>>>> in order to support the functionality of this patch, QEMU
>>>>>>> needs to query for this capability and set the
>>>>>>> "hcall-multi-tce" hypertas property only if the capability is
>>>>>>> present, otherwise there will be serious performance
>>>>>>> degradation.
>>>>>>>
>>>>>>> Cc: David Gibson<[email-masked]>  Signed-off-by:
>>>>>>> Alexey Kardashevskiy<[email-masked]>  Signed-off-by: Paul
>>>>>>> Mackerras<[email-masked]>
>>>>>> Only a few minor nits. Ben already commented on implementation
>>>>>> details.
>>>>>>
>>>>>>> --- Changelog: 2013/06/05: * fixed mistype about IBMVIO in the
>>>>>>> commit message * updated doc and moved it to another section *
>>>>>>> changed capability number
>>>>>>>
>>>>>>> 2013/05/21: * added kvm_vcpu_arch::tce_tmp * removed cleanup
>>>>>>> if put_indirect failed, instead we do not even start writing
>>>>>>> to TCE table if we cannot get TCEs from the user and they are
>>>>>>> invalid * kvmppc_emulated_h_put_tce is split to
>>>>>>> kvmppc_emulated_put_tce and kvmppc_emulated_validate_tce (for
>>>>>>> the previous item) * fixed bug with failthrough for H_IPI *
>>>>>>> removed all get_user() from real mode handlers *
>>>>>>> kvmppc_lookup_pte() added (instead of making lookup_linux_pte
>>>>>>> public) --- Documentation/virtual/kvm/api.txt       |   17 ++
>>>>>>> arch/powerpc/include/asm/kvm_host.h     |    2 +
>>>>>>> arch/powerpc/include/asm/kvm_ppc.h      |   16 +-
>>>>>>> arch/powerpc/kvm/book3s_64_vio.c        |  118 ++++++++++++++
>>>>>>> arch/powerpc/kvm/book3s_64_vio_hv.c     |  266
>>>>>>> +++++++++++++++++++++++++++---- arch/powerpc/kvm/book3s_hv.c
>>>>>>> |   39 +++++ arch/powerpc/kvm/book3s_hv_rmhandlers.S |    6 +
>>>>>>> arch/powerpc/kvm/book3s_pr_papr.c       |   37 ++++-
>>>>>>> arch/powerpc/kvm/powerpc.c              |    3 +
>>>>>>> include/uapi/linux/kvm.h                |    1 + 10 files
>>>>>>> changed, 473 insertions(+), 32 deletions(-)
>>>>>>>
>>>>>>> diff --git a/Documentation/virtual/kvm/api.txt
>>>>>>> b/Documentation/virtual/kvm/api.txt index 5f91eda..6c082ff
>>>>>>> 100644 --- a/Documentation/virtual/kvm/api.txt +++
>>>>>>> b/Documentation/virtual/kvm/api.txt @@ -2362,6 +2362,23 @@
>>>>>>> calls by the guest for that service will be passed to
>>>>>>> userspace to be handled.
>>>>>>>
>>>>>>>
>>>>>>> +4.83 KVM_CAP_PPC_MULTITCE + +Capability:
>>>>>>> KVM_CAP_PPC_MULTITCE +Architectures: ppc +Type: vm + +This
>>>>>>> capability tells the guest that multiple TCE entry add/remove
>>>>>>> hypercalls +handling is supported by the kernel. This
>>>>>>> significanly accelerates DMA +operations for PPC KVM guests.
>>>>>>> + +Unlike other capabilities in this section, this one does
>>>>>>> not have an ioctl. +Instead, when the capability is present,
>>>>>>> the H_PUT_TCE_INDIRECT and +H_STUFF_TCE hypercalls are to be
>>>>>>> handled in the host kernel and not passed to +the guest.
>>>>>>> Othwerwise it might be better for the guest to continue using
>>>>>>> H_PUT_TCE +hypercall (if KVM_CAP_SPAPR_TCE or
>>>>>>> KVM_CAP_SPAPR_TCE_IOMMU are present).
>>>>>> While this describes perfectly well what the consequences are of
>>>>>> the patches, it does not describe properly what the CAP actually
>>>>>> expresses. The CAP only says "this kernel is able to handle
>>>>>> H_PUT_TCE_INDIRECT and H_STUFF_TCE hypercalls directly". All
>>>>>> other consequences are nice to document, but the semantics of
>>>>>> the CAP are missing.
>>>>>
>>>>> ? It expresses ability to handle 2 hcalls. What is missing?
>>>> You don't describe the kvm<->  qemu interface. You describe some
>>>> decisions qemu can take from this cap.
>>>
>>> This file does not mention qemu at all. And the interface is - qemu
>>> (or kvmtool could do that) just adds "hcall-multi-tce" to
>>> "ibm,hypertas-functions" but this is for pseries linux and AIX could
>>> always do it (no idea about it). Does it really have to be in this
>>> file?
>> Ok, let's go back a step. What does this CAP describe? Don't look at the
>> description you wrote above. Just write a new one.
> The CAP means the kernel is capable of handling hcalls A and B without
> passing those into the user space. That accelerates DMA.
>
>
>> What exactly can user space expect when it finds this CAP?
> The user space can expect that its handlers for A and B are not going to be
> called if it configures the guest appropriately.
>
> Any better? :)

A lot, yes. This is what the CAP actually means.

It's nice to give some guidance in the documentation of implications 
(should expose "ibm,hypertas-functions" to enable the guest to actually 
use these for example) but the first paragraph should only indicate what 
the CAP changes.

Alex