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

From: Alexander Graf <[email-masked]>
Date: Mon, 17 Jun 2013 12:48:05 +0200

On 06/17/2013 12:46 PM, Alexander Graf wrote:
> 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.

Actually a nitpick here too. User space can expect that its handlers for 
A and B are going to already be processed by KVM. Regardless of how user 
space configures the guest.

Alex