[Qemu-devel] [PATCH] s390x: Enable KVM huge page backing support

[Qemu-devel] [PATCH] s390x: Enable KVM huge page backing support

[Janosch Frank]

From: Janosch Frank <frankja@linux.vnet.ibm.com>

QEMU has had huge page support for a longer time already, but KVM
memory management under s390x needed some changes to work with huge
backings.

Now that we have support, let's enable it if requested and
available. Otherwise we now properly tell the user if there is no
support and back out instead of failing to run the VM later on.

Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
---

Right now there's a conflict with arm for the capability, so I guess
I'll have to touch the cap number soonish. :)

---
 linux-headers/linux/kvm.h | 1 +
 target/s390x/kvm.c        | 6 ++++++
 2 files changed, 7 insertions(+)

diff --git a/linux-headers/linux/kvm.h b/linux-headers/linux/kvm.h
index 98f389a5a3..e3b1e3070c 100644
--- a/linux-headers/linux/kvm.h
+++ b/linux-headers/linux/kvm.h
@@ -949,6 +949,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_GET_MSR_FEATURES 153
 #define KVM_CAP_HYPERV_EVENTFD 154
 #define KVM_CAP_HYPERV_TLBFLUSH 155
+#define KVM_CAP_S390_HPAGE 156
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c
index d923cf4240..1ce0b42039 100644
--- a/target/s390x/kvm.c
+++ b/target/s390x/kvm.c
@@ -285,6 +285,12 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
 {
     MachineClass *mc = MACHINE_GET_CLASS(ms);
 
+    if (mem_path && kvm_vm_enable_cap(s, KVM_CAP_S390_HPAGE, 0)) {
+        error_report("Huge page backing was specified, "
+                     "but this KVM does not support huge pages.");
+        return -EINVAL;
+    }
+
     mc->default_cpu_type = S390_CPU_TYPE_NAME("host");
     cap_sync_regs = kvm_check_extension(s, KVM_CAP_SYNC_REGS);
     cap_async_pf = kvm_check_extension(s, KVM_CAP_ASYNC_PF);
-- 
2.14.3

Re: [Qemu-devel] [PATCH] s390x: Enable KVM huge page backing support

[Cornelia Huck]

On Tue, 31 Jul 2018 09:42:36 +0100
Janosch Frank <frankja@linux.ibm.com> wrote:

> From: Janosch Frank <frankja@linux.vnet.ibm.com>
> 
> QEMU has had huge page support for a longer time already, but KVM
> memory management under s390x needed some changes to work with huge
> backings.
> 
> Now that we have support, let's enable it if requested and
> available. Otherwise we now properly tell the user if there is no
> support and back out instead of failing to run the VM later on.
> 
> Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>

You may want to change author and signoff to the new, shorter address.

> ---
> 
> Right now there's a conflict with arm for the capability, so I guess
> I'll have to touch the cap number soonish. :)

You can fix this by spitting out the linux-headers update into a patch
that will be replaced with a proper headers update :)

> 
> ---
>  linux-headers/linux/kvm.h | 1 +
>  target/s390x/kvm.c        | 6 ++++++
>  2 files changed, 7 insertions(+)
> 
> diff --git a/linux-headers/linux/kvm.h b/linux-headers/linux/kvm.h
> index 98f389a5a3..e3b1e3070c 100644
> --- a/linux-headers/linux/kvm.h
> +++ b/linux-headers/linux/kvm.h
> @@ -949,6 +949,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_GET_MSR_FEATURES 153
>  #define KVM_CAP_HYPERV_EVENTFD 154
>  #define KVM_CAP_HYPERV_TLBFLUSH 155
> +#define KVM_CAP_S390_HPAGE 156
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  
> diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c
> index d923cf4240..1ce0b42039 100644
> --- a/target/s390x/kvm.c
> +++ b/target/s390x/kvm.c
> @@ -285,6 +285,12 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>  {
>      MachineClass *mc = MACHINE_GET_CLASS(ms);
>  
> +    if (mem_path && kvm_vm_enable_cap(s, KVM_CAP_S390_HPAGE, 0)) {
> +        error_report("Huge page backing was specified, "
> +                     "but this KVM does not support huge pages.");

Please drop the trailing '.', as by the documentation for error_report().

> +        return -EINVAL;
> +    }
> +
>      mc->default_cpu_type = S390_CPU_TYPE_NAME("host");
>      cap_sync_regs = kvm_check_extension(s, KVM_CAP_SYNC_REGS);
>      cap_async_pf = kvm_check_extension(s, KVM_CAP_ASYNC_PF);

Re: [Qemu-devel] [PATCH] s390x: Enable KVM huge page backing support

[David Hildenbrand]

On 31.07.2018 10:42, Janosch Frank wrote:
> From: Janosch Frank <frankja@linux.vnet.ibm.com>
> 
> QEMU has had huge page support for a longer time already, but KVM
> memory management under s390x needed some changes to work with huge
> backings.
> 
> Now that we have support, let's enable it if requested and
> available. Otherwise we now properly tell the user if there is no
> support and back out instead of failing to run the VM later on.
> 
> Signed-off-by: Janosch Frank <frankja@linux.vnet.ibm.com>
> ---
> 
> Right now there's a conflict with arm for the capability, so I guess
> I'll have to touch the cap number soonish. :)
> 
> ---
>  linux-headers/linux/kvm.h | 1 +
>  target/s390x/kvm.c        | 6 ++++++
>  2 files changed, 7 insertions(+)
> 
> diff --git a/linux-headers/linux/kvm.h b/linux-headers/linux/kvm.h
> index 98f389a5a3..e3b1e3070c 100644
> --- a/linux-headers/linux/kvm.h
> +++ b/linux-headers/linux/kvm.h
> @@ -949,6 +949,7 @@ struct kvm_ppc_resize_hpt {
>  #define KVM_CAP_GET_MSR_FEATURES 153
>  #define KVM_CAP_HYPERV_EVENTFD 154
>  #define KVM_CAP_HYPERV_TLBFLUSH 155
> +#define KVM_CAP_S390_HPAGE 156
>  
>  #ifdef KVM_CAP_IRQ_ROUTING
>  
> diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c
> index d923cf4240..1ce0b42039 100644
> --- a/target/s390x/kvm.c
> +++ b/target/s390x/kvm.c
> @@ -285,6 +285,12 @@ int kvm_arch_init(MachineState *ms, KVMState *s)
>  {
>      MachineClass *mc = MACHINE_GET_CLASS(ms);
>  
> +    if (mem_path && kvm_vm_enable_cap(s, KVM_CAP_S390_HPAGE, 0)) {
> +        error_report("Huge page backing was specified, "
> +                     "but this KVM does not support huge pages.");
> +        return -EINVAL;
> +    }
> +
>      mc->default_cpu_type = S390_CPU_TYPE_NAME("host");
>      cap_sync_regs = kvm_check_extension(s, KVM_CAP_SYNC_REGS);
>      cap_async_pf = kvm_check_extension(s, KVM_CAP_ASYNC_PF);
> 

Okay, I wonder if now is the right time to actually detect the pagesize
instead of relying on mempath pointing at hugetlbfs. I remember Power
already doing such a detection.

mempath is just an indicator.

-- 

Thanks,

David / dhildenb