Subject: Re: [PATCH 8/8] KVM: PPC: Add hugepage support for IOMMU in-kernel handling

From: Alexey Kardashevskiy <[email-masked]>
Date: Thu, 11 Jul 2013 18:57:27 +1000

On 07/10/2013 03:32 AM, Alexander Graf wrote:
> On 07/06/2013 05:07 PM, Alexey Kardashevskiy wrote:
>> This adds special support for huge pages (16MB).  The reference
>> counting cannot be easily done for such pages in real mode (when
>> MMU is off) so we added a list of huge pages.  It is populated in
>> virtual mode and get_page is called just once per a huge page.
>> Real mode handlers check if the requested page is huge and in the list,
>> then no reference counting is done, otherwise an exit to virtual mode
>> happens.  The list is released at KVM exit.  At the moment the fastest
>> card available for tests uses up to 9 huge pages so walking through this
>> list is not very expensive.  However this can change and we may want
>> to optimize this.
>>
>> Signed-off-by: Paul Mackerras<[email-masked]>
>> Signed-off-by: Alexey Kardashevskiy<[email-masked]>
>>
>> ---
>>
>> Changes:
>> 2013/06/27:
>> * list of huge pages replaces with hashtable for better performance
> 
> So the only thing your patch description really talks about is not true
> anymore?
> 
>> * spinlock removed from real mode and only protects insertion of new
>> huge [ages descriptors into the hashtable
>>
>> 2013/06/05:
>> * fixed compile error when CONFIG_IOMMU_API=n
>>
>> 2013/05/20:
>> * the real mode handler now searches for a huge page by gpa (used to be pte)
>> * the virtual mode handler prints warning if it is called twice for the same
>> huge page as the real mode handler is expected to fail just once - when a
>> huge
>> page is not in the list yet.
>> * the huge page is refcounted twice - when added to the hugepage list and
>> when used in the virtual mode hcall handler (can be optimized but it will
>> make the patch less nice).
>>
>> Signed-off-by: Alexey Kardashevskiy<[email-masked]>
>> ---
>>   arch/powerpc/include/asm/kvm_host.h |  25 +++++++++
>>   arch/powerpc/kernel/iommu.c         |   6 ++-
>>   arch/powerpc/kvm/book3s_64_vio.c    | 104
>> +++++++++++++++++++++++++++++++++---
>>   arch/powerpc/kvm/book3s_64_vio_hv.c |  21 ++++++--
>>   4 files changed, 146 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/powerpc/include/asm/kvm_host.h
>> b/arch/powerpc/include/asm/kvm_host.h
>> index 53e61b2..a7508cf 100644
>> --- a/arch/powerpc/include/asm/kvm_host.h
>> +++ b/arch/powerpc/include/asm/kvm_host.h
>> @@ -30,6 +30,7 @@
>>   #include<linux/kvm_para.h>
>>   #include<linux/list.h>
>>   #include<linux/atomic.h>
>> +#include<linux/hashtable.h>
>>   #include<asm/kvm_asm.h>
>>   #include<asm/processor.h>
>>   #include<asm/page.h>
>> @@ -182,10 +183,34 @@ struct kvmppc_spapr_tce_table {
>>       u32 window_size;
>>       struct iommu_group *grp;        /* used for IOMMU groups */
>>       struct vfio_group *vfio_grp;        /* used for IOMMU groups */
>> +    DECLARE_HASHTABLE(hash_tab, ilog2(64));    /* used for IOMMU groups */
>> +    spinlock_t hugepages_write_lock;    /* used for IOMMU groups */
>>       struct { struct { unsigned long put, indir, stuff; } rm, vm; } stat;
>>       struct page *pages[0];
>>   };
>>
>> +/*
>> + * The KVM guest can be backed with 16MB pages.
>> + * In this case, we cannot do page counting from the real mode
>> + * as the compound pages are used - they are linked in a list
>> + * with pointers as virtual addresses which are inaccessible
>> + * in real mode.
>> + *
>> + * The code below keeps a 16MB pages list and uses page struct
>> + * in real mode if it is already locked in RAM and inserted into
>> + * the list or switches to the virtual mode where it can be
>> + * handled in a usual manner.
>> + */
>> +#define KVMPPC_SPAPR_HUGEPAGE_HASH(gpa)    hash_32(gpa>>  24, 32)
>> +
>> +struct kvmppc_spapr_iommu_hugepage {
>> +    struct hlist_node hash_node;
>> +    unsigned long gpa;    /* Guest physical address */
>> +    unsigned long hpa;    /* Host physical address */
>> +    struct page *page;    /* page struct of the very first subpage */
>> +    unsigned long size;    /* Huge page size (always 16MB at the moment) */
>> +};
>> +
>>   struct kvmppc_linear_info {
>>       void        *base_virt;
>>       unsigned long     base_pfn;
>> diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
>> index 51678ec..e0b6eca 100644
>> --- a/arch/powerpc/kernel/iommu.c
>> +++ b/arch/powerpc/kernel/iommu.c
>> @@ -999,7 +999,8 @@ int iommu_free_tces(struct iommu_table *tbl, unsigned
>> long entry,
>>               if (!pg) {
>>                   ret = -EAGAIN;
>>               } else if (PageCompound(pg)) {
>> -                ret = -EAGAIN;
>> +                /* Hugepages will be released at KVM exit */
>> +                ret = 0;
>>               } else {
>>                   if (oldtce&  TCE_PCI_WRITE)
>>                       SetPageDirty(pg);
>> @@ -1009,6 +1010,9 @@ int iommu_free_tces(struct iommu_table *tbl,
>> unsigned long entry,
>>               struct page *pg = pfn_to_page(oldtce>>  PAGE_SHIFT);
>>               if (!pg) {
>>                   ret = -EAGAIN;
>> +            } else if (PageCompound(pg)) {
>> +                /* Hugepages will be released at KVM exit */
>> +                ret = 0;
>>               } else {
>>                   if (oldtce&  TCE_PCI_WRITE)
>>                       SetPageDirty(pg);
>> diff --git a/arch/powerpc/kvm/book3s_64_vio.c
>> b/arch/powerpc/kvm/book3s_64_vio.c
>> index 2b51f4a..c037219 100644
>> --- a/arch/powerpc/kvm/book3s_64_vio.c
>> +++ b/arch/powerpc/kvm/book3s_64_vio.c
>> @@ -46,6 +46,40 @@
>>
>>   #define ERROR_ADDR      ((void *)~(unsigned long)0x0)
>>
>> +#ifdef CONFIG_IOMMU_API
> 
> Can't you just make CONFIG_IOMMU_API mandatory in Kconfig?

Where exactly (it is rather SPAPR_TCE_IOMMU but does not really matter)?
Select it on KVM_BOOK3S_64? CONFIG_KVM_BOOK3S_64_HV?
CONFIG_KVM_BOOK3S_64_PR? PPC_BOOK3S_64?

I am trying to imagine a configuration where we really do not want
IOMMU_API. Ben mentioned PPC32 and embedded PPC64 and that's it so any of
BOOK3S (KVM_BOOK3S_64 is the best) should be fine, no?

-- 
Alexey