Re: [PATCH v2 2/2] accel: kvm: Add aligment assert for kvm_log_clear_one_slot

[Thomas Huth]

On 09/03/2021 15.05, Keqian Zhu wrote:
> On 2021/3/9 21:48, Thomas Huth wrote:
> > On 17/12/2020 02.49, Keqian Zhu wrote:
> > > The parameters start and size are transfered from QEMU memory
> > > emulation layer. It can promise that they are TARGET_PAGE_SIZE
> > > aligned. However, KVM needs they are qemu_real_page_size aligned.
> > >
> > > Though no caller breaks this aligned requirement currently, we'd
> > > better add an explicit assert to avoid future breaking.
> > >
> > > Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
> > > ---
> > >    accel/kvm/kvm-all.c | 7 +++++++
> > >    1 file changed, 7 insertions(+)
> > >
> > > ---
> > > v2
> > >    - Address Andrew's commment (Use assert instead of return err).
> > >
> > > diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c
> > > index f6b16a8df8..73b195cc41 100644
> > > --- a/accel/kvm/kvm-all.c
> > > +++ b/accel/kvm/kvm-all.c
> > > @@ -692,6 +692,10 @@ out:
> > >    #define KVM_CLEAR_LOG_ALIGN  (qemu_real_host_page_size << 
> > > KVM_CLEAR_LOG_SHIFT)
> > >    #define KVM_CLEAR_LOG_MASK   (-KVM_CLEAR_LOG_ALIGN)
> > >    +/*
> > > + * As the granule of kvm dirty log is qemu_real_host_page_size,
> > > + * @start and @size are expected and restricted to align to it.
> > > + */
> > >    static int kvm_log_clear_one_slot(KVMSlot *mem, int as_id, uint64_t start,
> > >                                      uint64_t size)
> > >    {
> > > @@ -701,6 +705,9 @@ static int kvm_log_clear_one_slot(KVMSlot *mem, int as_id, 
> > > uint64_t start,
> > >        unsigned long *bmap_clear = NULL, psize = qemu_real_host_page_size;
> > >        int ret;
> > >    +    /* Make sure start and size are qemu_real_host_page_size aligned */
> > > +    assert(QEMU_IS_ALIGNED(start | size, psize));
> >
> > Sorry, but that was a bad idea: It triggers and kills my Centos 6 VM:
> >
> > $ qemu-system-x86_64 -accel kvm -hda ~/virt/images/centos6.qcow2 -m 1G
> > qemu-system-x86_64: ../../devel/qemu/accel/kvm/kvm-all.c:690: 
> > kvm_log_clear_one_slot: Assertion `QEMU_IS_ALIGNED(start | size, psize)' failed.
> > Aborted (core dumped)
> Hi Thomas,
>
> I think this patch is ok, maybe it trigger a potential bug?

Well, sure, there is either a bug somewhere else or in this new code. But it's 
certainly not normal that the assert() triggers, is it?

FWIW, here's a backtrace:

#0  0x00007ffff2c1584f in raise () at /lib64/libc.so.6
#1  0x00007ffff2bffc45 in abort () at /lib64/libc.so.6
#2  0x00007ffff2bffb19 in _nl_load_domain.cold.0 () at /lib64/libc.so.6
#3  0x00007ffff2c0de36 in .annobin_assert.c_end () at /lib64/libc.so.6
#4  0x0000555555ba25f3 in kvm_log_clear_one_slot
    (size=6910080, start=0, as_id=0, mem=0x555556e1ee00)
    at ../../devel/qemu/accel/kvm/kvm-all.c:691
#5  0x0000555555ba25f3 in kvm_physical_log_clear
    (section=0x7fffffffd0b0, section=0x7fffffffd0b0, kml=0x555556dbaac0)
    at ../../devel/qemu/accel/kvm/kvm-all.c:843
#6  0x0000555555ba25f3 in kvm_log_clear (listener=0x555556dbaac0, 
section=0x7fffffffd0b0)
    at ../../devel/qemu/accel/kvm/kvm-all.c:1253
#7  0x0000555555b023d8 in memory_region_clear_dirty_bitmap
    (mr=mr@entry=0x5555573394c0, start=start@entry=0, len=len@entry=6910080)
    at ../../devel/qemu/softmmu/memory.c:2132
#8  0x0000555555b313d9 in cpu_physical_memory_snapshot_and_clear_dirty
    (mr=mr@entry=0x5555573394c0, offset=offset@entry=0, 
length=length@entry=6910080, client=client@entry=0) at 
../../devel/qemu/softmmu/physmem.c:1109
#9  0x0000555555b02483 in memory_region_snapshot_and_clear_dirty
    (mr=mr@entry=0x5555573394c0, addr=addr@entry=0, size=size@entry=6910080, 
client=client@entry=0)
    at ../../devel/qemu/softmmu/memory.c:2146
#10 0x0000555555babe99 in vga_draw_graphic (full_update=0, s=0x5555573394b0)
    at ../../devel/qemu/hw/display/vga.c:1661
#11 0x0000555555babe99 in vga_update_display (opaque=0x5555573394b0)
    at ../../devel/qemu/hw/display/vga.c:1784
#12 0x0000555555babe99 in vga_update_display (opaque=0x5555573394b0)
    at ../../devel/qemu/hw/display/vga.c:1757
#13 0x00005555558ddd32 in graphic_hw_update (con=0x555556a11800)
    at ../../devel/qemu/ui/console.c:279
#14 0x00005555558dccd2 in dpy_refresh (s=0x555556c17da0) at 
../../devel/qemu/ui/console.c:1742
#15 0x00005555558dccd2 in gui_update (opaque=opaque@entry=0x555556c17da0)
    at ../../devel/qemu/ui/console.c:209
#16 0x0000555555dbd520 in timerlist_run_timers (timer_list=0x555556937c50)
    at ../../devel/qemu/util/qemu-timer.c:574
#17 0x0000555555dbd520 in timerlist_run_timers (timer_list=0x555556937c50)
    at ../../devel/qemu/util/qemu-timer.c:499
#18 0x0000555555dbd74a in qemu_clock_run_timers (type=<optimized out>)
    at ../../devel/qemu/util/qemu-timer.c:670
#19 0x0000555555dbd74a in qemu_clock_run_all_timers () at 
../../devel/qemu/util/qemu-timer.c:670

Looks like something in the vga code calls this with size=6910080
and thus triggers the alignment assertion?

 Thomas