Re: [PATCH 4/6] KVM: selftests: memslot_perf_test: Support variable guest page size

[Gavin Shan]

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@xxxxxxxxxx>
> > > > > ---
> > > > >   .../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