Re: [Qemu-devel] [PATCH 2/8] s390: autodetect map private

[Alexander Graf <agraf@suse.de>]

Jens Freimann wrote:
> From: Christian Borntraeger <borntraeger@de.ibm.com>
>
> By default qemu will use MAP_PRIVATE for guest pages. This will write
> protect pages and thus break on s390 systems that dont support this feature.
> Therefore qemu has a hack to always use MAP_SHARED for s390. But MAP_SHARED
> has other problems (no dirty pages tracking, a lot more swap overhead etc.)
> Newer systems allow the distinction via KVM_CAP_S390_COW. With this feature
> qemu can use the standard qemu alloc if available, otherwise it will use
> the old s390 hack.
>
> Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
> Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
> ---
>  exec.c             |   54 +++++++++++++++++++++++++++++++++++++---------------
>  kvm.h              |    9 +++++++++
>  oslib-posix.c      |    3 +++
>  target-s390x/kvm.c |    6 ++++++
>  4 files changed, 57 insertions(+), 15 deletions(-)
>
> diff --git a/exec.c b/exec.c
> index a0494c7..8fec680 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -2618,6 +2618,43 @@ void qemu_ram_set_idstr(ram_addr_t addr, const char *name, DeviceState *dev)
>      }
>  }
>  
> +/*
> + * lets make sure that we dont have the old s390x limitations regarding
> + * guest mappings
> + */
> +static int legacy_s390x_mem_layout(void)
> +{
> +#if defined(TARGET_S390X)
> +    return kvm_has_legacy_s390x_memlayout();
> +#else
> +    return 0;
> +#endif
> +}
> +
> +/*
> + * Legacy layout for s390:
> + * Older S390 KVM requires the topmost vma of the RAM to be
> + * smaller than an system defined value, which is at least 256GB.
> + * Larger systems have larger values. We put the guest between
> + * the end of data segment (system break) and this value. We
> + * use 32GB as a base to have enough room for the system break
> + * to grow. We also have to use MAP parameters that avoid
> + * read-only mapping of guest pages.
> + */
> +static void *legacy_s390_alloc(ram_addr_t size)
> +{
> +    void *mem;
> +
> +    mem = mmap((void *) 0x800000000ULL, size,
> +               PROT_EXEC|PROT_READ|PROT_WRITE,
> +               MAP_SHARED | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
> +    if (mem == MAP_FAILED) {
> +        fprintf(stderr, "Allocating RAM failed\n");
> +        abort();
> +    }
> +    return mem;
> +}
> +
>   

Is there any way we can move the above code to target-s390x? Having the
branch below is already invasive enough for generic code, but we really
don't need all the special s390 quirks to live here.

>  ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
>                                     MemoryRegion *mr)
>  {
> @@ -2644,26 +2681,13 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
>              exit(1);
>  #endif
>          } else {
> -#if defined(TARGET_S390X) && defined(CONFIG_KVM)
> -            /* S390 KVM requires the topmost vma of the RAM to be smaller than
> -               an system defined value, which is at least 256GB. Larger systems
> -               have larger values. We put the guest between the end of data
> -               segment (system break) and this value. We use 32GB as a base to
> -               have enough room for the system break to grow. */
> -            new_block->host = mmap((void*)0x800000000, size,
> -                                   PROT_EXEC|PROT_READ|PROT_WRITE,
> -                                   MAP_SHARED | MAP_ANONYMOUS | MAP_FIXED, -1, 0);
> -            if (new_block->host == MAP_FAILED) {
> -                fprintf(stderr, "Allocating RAM failed\n");
> -                abort();
> -            }
> -#else
>              if (xen_enabled()) {
>                  xen_ram_alloc(new_block->offset, size, mr);
>   
#ifdef TARGET_S390X
> +            } else if (legacy_s390x_mem_layout()) {
> +                new_block->host = legacy_s390_alloc(size);
>   
#endif

maybe? That way you could move everything to target-s390x.
>              } else {
>                  new_block->host = qemu_vmalloc(size);
>              }
> -#endif
>              qemu_madvise(new_block->host, size, QEMU_MADV_MERGEABLE);
>          }
>      }
> diff --git a/kvm.h b/kvm.h
> index 9c7b0ea..ca0557e 100644
> --- a/kvm.h
> +++ b/kvm.h
> @@ -62,6 +62,15 @@ int kvm_has_pit_state2(void);
>  int kvm_has_many_ioeventfds(void);
>  int kvm_has_gsi_routing(void);
>  
> +#ifndef CONFIG_KVM 
> +static inline int kvm_has_legacy_s390x_memlayout(void)
>   

An s390 function in generic kvm.h? No way :)

> +{
> +    return 0;
> +}
> +#else
> +int kvm_has_legacy_s390x_memlayout(void);
> +#endif
> +
>  int kvm_allows_irq0_override(void);
>  
>  #ifdef NEED_CPU_H
> diff --git a/oslib-posix.c b/oslib-posix.c
> index b6a3c7f..93902ac 100644
> --- a/oslib-posix.c
> +++ b/oslib-posix.c
> @@ -41,6 +41,9 @@ extern int daemon(int, int);
>        therefore we need special code which handles running on Valgrind. */
>  #  define QEMU_VMALLOC_ALIGN (512 * 4096)
>  #  define CONFIG_VALGRIND
> +#elif defined(__linux__) && defined(__s390x__)
> +   /* Use 1 MiB (segment size) alignment so gmap can be used by KVM. */
> +#  define QEMU_VMALLOC_ALIGN (256 * 4096)
>  #else
>  #  define QEMU_VMALLOC_ALIGN getpagesize()
>  #endif
> diff --git a/target-s390x/kvm.c b/target-s390x/kvm.c
> index 90aad61..93a8431 100644
> --- a/target-s390x/kvm.c
> +++ b/target-s390x/kvm.c
> @@ -135,6 +135,12 @@ int kvm_arch_get_registers(CPUS390XState *env)
>      return 0;
>  }
>  
> +int kvm_has_legacy_s390x_memlayout(void)
> +{
> +    return !kvm_check_extension(kvm_state, KVM_CAP_S390_GMAP) ||
> +           !kvm_check_extension(kvm_state, KVM_CAP_S390_COW);
> +}
> +
>  int kvm_arch_insert_sw_breakpoint(CPUS390XState *env, struct kvm_sw_breakpoint *bp)
>  {
>      static const uint8_t diag_501[] = {0x83, 0x24, 0x05, 0x01};
>