On 25.01.19 10:23, Cornelia Huck wrote:
> On Thu, 24 Jan 2019 17:57:56 +0100
> Igor Mammedov <imamm...@redhat.com> wrote:
>
>> I plan to deprecate -mem-path option and replace it with memory-backend,
>> for that it's necessary to get rid of mem_path global variable.
>> Do it for s390x case, replacing it with alternative way to enable
>> 1Mb hugepages capability.
>
> Getting rid of accessing mem_path directly sounds good.
>
>>
>> Signed-off-by: Igor Mammedov <imamm...@redhat.com>
>> ---
>> PS:
>> Original code nor the new one probably is not entirely correct when
>> huge pages are enabled in case where mixed initial RAM and memory
>> backends are used, backend's page size might not match initial RAM's
>> so I'm not sure if enabling 1MB cap is correct in this case on s390
>> (should it be the same for all RAM???).
>> With new approach 1Mb cap is not enabled if the smallest page size
>> is not 1Mb.
>> ---
>> target/s390x/kvm.c | 37 ++++++++++++++++---------------------
>> 1 file changed, 16 insertions(+), 21 deletions(-)
>>
>> diff --git a/target/s390x/kvm.c b/target/s390x/kvm.c
>> index 2ebf26a..22e868a 100644
>> --- a/target/s390x/kvm.c
>> +++ b/target/s390x/kvm.c
>> @@ -285,33 +285,28 @@ void kvm_s390_crypto_reset(void)
>> }
>> }
>>
>> -static int kvm_s390_configure_mempath_backing(KVMState *s)
>> +static int kvm_s390_configure_hugepage_backing(KVMState *s)
>> {
>> - size_t path_psize = qemu_mempath_getpagesize(mem_path);
>> + size_t psize = qemu_getrampagesize();
>>
>> - if (path_psize == 4 * KiB) {
>> - return 0;
>> - }
>> -
>> - if (!hpage_1m_allowed()) {
>> - error_report("This QEMU machine does not support huge page "
>> - "mappings");
>> - return -EINVAL;
>> - }
>> + if (psize == 1 * MiB) {
>> + if (!hpage_1m_allowed()) {
>> + error_report("This QEMU machine does not support huge page "
>> + "mappings");
>> + return -EINVAL;
>> + }
>>
>> - if (path_psize != 1 * MiB) {
>> + 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;
>> + } else if (psize == 2 * GiB) {
>> 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;
>
> Just to compare, the old code did:
> - 4K pages -> all fine, do nothing
> - 1MB pages not allowed -> get out, regardless of the actual huge page
> size
> - 1MB pages -> try to enable, if possible
> - all other sizes -> moan about 2G pages and get out
>
> And the new code does:
> - 1M pages -> get out if 1MB not allowed, otherwise try to enable
> - 2G pages -> moan about 2G pages and get out
> - all other sizes -> all fine, do nothing
>
> So, now the user will:
> - get a different error if they try to run with a 2G backing but
> hpage_1m_allowed is off (which does not sound like a problem to me)
> - get the all-clear if they specified a hypothetical different page
> size, while the code always complained about 2G pages before
>
> Are there any chances at all that there may Yet Another Size? If not,
> this looks fine.
I think the next logical step is 1TB pages - unlikely for the next years ;)
--
Thanks,
David / dhildenb
