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

[Alexander Graf <agraf@suse.de>]

On 06/12/2012 01:20 PM, Christian Borntraeger wrote:
>> 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.
>
> Hmm, we have to have a special hook somehow.
> What about this approach?
>
> -----------------------
>
> 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>
> ---
>   exec.c             |   19 ++++---------------
>   kvm.h              |    1 +
>   oslib-posix.c      |    3 +++
>   target-s390x/kvm.c |   34 ++++++++++++++++++++++++++++++++++
>   4 files changed, 42 insertions(+), 15 deletions(-)
>
> diff --git a/exec.c b/exec.c
> index a587e7a..9b9b8e1 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -2645,26 +2645,15 @@ 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);
> +#if defined(TARGET_S390X)
> +            } else if (kvm_enabled()) {
> +                new_block->host = kvm_arch_alloc(size);

Since it lives in an s390 specific branch, the function name should 
probably be called s390 specific. If we ever need another architecture 
to have a kvm specific ram allocator, we can make it generic when that 
time comes. Until then, let's treat s390 as the oddball it is :).

Apart from that, this approach looks a lot nicer, yes.

Alex

> +#endif
>               } 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..9d50016 100644
> --- a/kvm.h
> +++ b/kvm.h
> @@ -102,6 +102,7 @@ int kvm_vcpu_ioctl(CPUArchState *env, int type, ...);
>
>   extern const KVMCapabilityInfo kvm_arch_required_capabilities[];
>
> +void *kvm_arch_alloc(ram_addr_t size);
>   void kvm_arch_pre_run(CPUArchState *env, struct kvm_run *run);
>   void kvm_arch_post_run(CPUArchState *env, struct kvm_run *run);
>
> 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..ccf5daa 100644
> --- a/target-s390x/kvm.c
> +++ b/target-s390x/kvm.c
> @@ -135,6 +135,40 @@ int kvm_arch_get_registers(CPUS390XState *env)
>       return 0;
>   }
>
> +/*
> + * 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;
> +}
> +
> +void *kvm_arch_alloc(ram_addr_t size)
> +{
> +    if (kvm_check_extension(kvm_state, KVM_CAP_S390_GMAP)&&
> +        kvm_check_extension(kvm_state, KVM_CAP_S390_COW)) {
> +        return qemu_vmalloc(size);
> +    } else {
> +        return legacy_s390_alloc(size);
> +    }
> +}
> +
>   int kvm_arch_insert_sw_breakpoint(CPUS390XState *env, struct kvm_sw_breakpoint *bp)
>   {
>       static const uint8_t diag_501[] = {0x83, 0x24, 0x05, 0x01};
>