Re: [qemu-s390x] [PATCH v4 2/2] s390x: Enable KVM huge page backing support

[David Hildenbrand]

On 01.08.2018 15:19, Janosch Frank wrote:
> 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 <address@hidden>
> ---
>  target/s390x/kvm.c | 34 ++++++++++++++++++++++++++++++++--
>  1 file changed, 32 insertions(+), 2 deletions(-)
> 
> diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c
> index d923cf4240..d1c72975cf 100644
> --- a/target/s390x/kvm.c
> +++ b/target/s390x/kvm.c
> @@ -34,6 +34,8 @@
>  #include "qapi/error.h"
>  #include "qemu/error-report.h"
>  #include "qemu/timer.h"
> +#include "qemu/units.h"
> +#include "qemu/mmap-alloc.h"
>  #include "sysemu/sysemu.h"
>  #include "sysemu/hw_accel.h"
>  #include "hw/hw.h"
> @@ -139,6 +141,7 @@ static int cap_mem_op;
>  static int cap_s390_irq;
>  static int cap_ri;
>  static int cap_gs;
> +static int cap_hpage_1m;
>  
>  static int active_cmma;
>  
> @@ -220,9 +223,9 @@ static void kvm_s390_enable_cmma(void)
>          .attr = KVM_S390_VM_MEM_ENABLE_CMMA,
>      };
>  
> -    if (mem_path) {
> +    if (cap_hpage_1m) {
>          warn_report("CMM will not be enabled because it is not "
> -                    "compatible with hugetlbfs.");
> +                    "compatible with huge memory backings.");

This implies some change to certain setups (tmpfs), but as they are
absolutely not common (and never really were supported/expected to
work), I think this is fine.

Reviewed-by: David Hildenbrand <address@hidden>

>          return;
>      }
>      rc = kvm_vm_ioctl(kvm_state, KVM_SET_DEVICE_ATTR, &attr);
> @@ -281,10 +284,37 @@ void kvm_s390_crypto_reset(void)
>      }
>  }
>  
> +static int kvm_s390_configure_mempath_backing(KVMState *s)
> +{
> +    size_t path_psize = qemu_mempath_getpagesize(mem_path);
> +
> +    if (path_psize == 4 * KiB)
> +        return 0;
> +
> +    if (path_psize != 1 * MiB) {
> +        error_report("Memory backing with 2G pages was specified, "
> +                     "but KVM does not support this memory backing");
> +        return -EINVAL;
> +    }
> +
> +    if (kvm_vm_enable_cap(s, KVM_CAP_S390_HPAGE_1M, 0)) {
> +        error_report("Memory backing with 1M pages was specified, "
> +                     "but KVM does not support this memory backing");
> +        return -EINVAL;
> +    }
> +
> +    cap_hpage_1m = 1;
> +    return 0;
> +}
> +
>  int kvm_arch_init(MachineState *ms, KVMState *s)
>  {
>      MachineClass *mc = MACHINE_GET_CLASS(ms);
>  
> +    if (mem_path && kvm_s390_configure_mempath_backing(s)) {
> +        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);
> 


-- 

Thanks,

David / dhildenb