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Reply-To: Gavin Shan <gshan@redhat.com>
Subject: Re: [PATCH 4/6] KVM: selftests: memslot_perf_test: Support variable
 guest page size
To:     "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
Cc:     kvm@vger.kernel.org, maz@kernel.org, linux-kernel@vger.kernel.org,
        zhenyzha@redhat.com, shan.gavin@gmail.com, kvmarm@lists.linux.dev,
        pbonzini@redhat.com, shuah@kernel.org,
        kvmarm@lists.cs.columbia.edu, ajones@ventanamicro.com
References: <20221014071914.227134-1-gshan@redhat.com>
 <20221014071914.227134-5-gshan@redhat.com>
 <3eecebca-a526-d10a-02d3-496ce919d577@maciej.szmigiero.name>
 <bd5df92c-6870-8053-0b35-a2ad993970bd@redhat.com>
 <da2b7db0-509a-c9e0-c36b-6487a265a779@redhat.com>
 <a1a8664c-4d06-89e7-8cfa-b730969bb841@maciej.szmigiero.name>
From:   Gavin Shan <gshan@redhat.com>
Message-ID: <5bfbe050-a654-8400-e1f1-dcfa4dba13e6@redhat.com>
Date:   Wed, 19 Oct 2022 08:26:19 +0800
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On 10/18/22 11:56 PM, Maciej S. Szmigiero wrote:
> On 18.10.2022 02:51, Gavin Shan wrote:
>> On 10/18/22 8:46 AM, Gavin Shan wrote:
>>> On 10/18/22 5:31 AM, Maciej S. Szmigiero wrote:
>>>> On 14.10.2022 09:19, Gavin Shan wrote:
>>>>> The test case is obviously broken on aarch64 because non-4KB guest
>>>>> page size is supported. The guest page size on aarch64 could be 4KB,
>>>>> 16KB or 64KB.
>>>>>
>>>>> This supports variable guest page size, mostly for aarch64.
>>>>>
>>>>>    - The host determines the guest page size when virtual machine is
>>>>>      created. The value is also passed to guest through the synchronization
>>>>>      area.
>>>>>
>>>>>    - The number of guest pages are unknown until the virtual machine
>>>>>      is to be created. So all the related macros are dropped. Instead,
>>>>>      their values are dynamically calculated based on the guest page
>>>>>      size.
>>>>>
>>>>>    - The static checks on memory sizes and pages becomes dependent
>>>>>      on guest page size, which is unknown until the virtual machine
>>>>>      is about to be created. So all the static checks are converted
>>>>>      to dynamic checks, done in check_memory_sizes().
>>>>>
>>>>>    - As the address passed to madvise() should be aligned to host page,
>>>>>      the size of page chunk is automatically selected, other than one
>>>>>      page.
>>>>>
>>>>>    - All other changes included in this patch are almost mechanical
>>>>>      replacing '4096' with 'guest_page_size'.
>>>>>
>>>>> Signed-off-by: Gavin Shan <gshan@redhat.com>
>>>>> ---
>>>>>   .../testing/selftests/kvm/memslot_perf_test.c | 191 +++++++++++-------
>>>>>   1 file changed, 115 insertions(+), 76 deletions(-)
>>>>>
>>>>> diff --git a/tools/testing/selftests/kvm/memslot_perf_test.c b/tools/testing/selftests/kvm/memslot_perf_test.c
>>>>> index d5aa9148f96f..d587bd952ff9 100644
>>>>> --- a/tools/testing/selftests/kvm/memslot_perf_test.c
>>>>> +++ b/tools/testing/selftests/kvm/memslot_perf_test.c
> (...)
>>>>> @@ -77,8 +61,7 @@ static_assert(MEM_TEST_UNMAP_SIZE_PAGES %
>>>>>    * for the total size of 25 pages.
>>>>>    * Hence, the maximum size here is 50 pages.
>>>>>    */
>>>>> -#define MEM_TEST_MOVE_SIZE_PAGES    (50)
>>>>> -#define MEM_TEST_MOVE_SIZE        (MEM_TEST_MOVE_SIZE_PAGES * 4096)
>>>>> +#define MEM_TEST_MOVE_SIZE        0x32000
>>>>
>>>> The above number seems less readable than an explicit value of 50 pages.
>>>>
>>>> In addition to that, it's 50 pages only with 4k page size, so at least
>>>> the comment above needs to be updated to reflect this fact.
>>>>
>>>
>>> Yeah, I will change the comments like below in next revision.
>>>
>>>   /*
>>>    * When running this test with 32k memslots, actually 32763 excluding
>>>    * the reserved memory slot 0, the memory for each slot is 0x4000 bytes.
>>>    * The last slot contains 0x19000 bytes memory. Hence, the maximum size
>>>    * here is 0x32000 bytes.
>>>    */
>>>
>>
>> I will replace those numbers with readable ones like below :)
>>
>> /*
>>   * When running this test with 32k memslots, actually 32763 excluding
>>   * the reserved memory slot 0, the memory for each slot is 16KB. The
>>   * last slot contains 100KB memory with the remaining 84KB. Hence,
>>   * the maximum size is double of that (200KB)
>>   */
> 
> Still, these numbers are for x86, which has KVM_INTERNAL_MEM_SLOTS
> defined as 3.
> 
> As far as I can see aarch64 has KVM_INTERNAL_MEM_SLOTS equal to 0, so
> this arch has 32766 slot available for the test memory.
> 
> Quick calculations show that this will result in 112 KiB of memory in
> the last slot for 4 KiB page size (while for 64 KiB page size the
> maximum slot count for this test is 8192 anyway - not counting slot 0).
> 

It seems your calculation had (512MB+64KB), instead of (512MB+4KB).
In this particular patch, we still have (512MB+4KB). How about to change
like below in this patch. In next patch, it's adjusted accordingly after
we have (512MB+64KB).

(1) In this patch, the comment is changed to as below

     /*
      * We have different number of memory slots, excluding the reserved
      * memory slot 0, on various architectures and configurations. The
      * memory size in this test is calculated by doubling the maximal
      * memory size in last memory slot, with alignment to the largest
      * supported page size (64KB).
      *
      * architecture   slots    memory-per-slot    memory-on-last-slot
      * --------------------------------------------------------------
      * x86-4KB        32763    16KB               100KB
      * arm64-4KB      32766    16KB               52KB
      * arm64-64KB     8192     64KB               64KB
      */
     #define MEM_TEST_MOVE_SIZE	0x40000           /* 256KB */

(2) In the next patch, where we have (512MB+64KB) after the various
     memory sizes are consolidated, It is adjusted accordingly as below.

     /*
      * We have different number of memory slots, excluding the reserved
      * memory slot 0, on various architectures and configurations. The
      * memory size in this test is calculated by doubling the maximal
      * memory size in last memory slot, with alignment to the largest
      * supported page size (64KB).
      *
      * architecture   slots    memory-per-slot    memory-on-last-slot
      * --------------------------------------------------------------
      * x86-4KB        32763    16KB               160KB
      * arm64-4KB      32766    16KB               112KB
      * arm64-64KB     8192     64KB               128KB
      */
     #define MEM_TEST_MOVE_SIZE	0x50000           /* 320KB */

Thanks,
Gavin