Re: [PATCH v2 0/3] x86/vdso: Add Hyper-V TSC page clocksource support

[Andy Lutomirski]

On Tue, Feb 14, 2017 at 7:50 AM, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
> Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes:
>
>> On Tue, 14 Feb 2017, Vitaly Kuznetsov wrote:
>>
>>> Hi,
>>>
>>> while we're still waiting for a definitive ACK from Microsoft that the
>>> algorithm is good for SMP case (as we can't prevent the code in vdso from
>>> migrating between CPUs) I'd like to send v2 with some modifications to keep
>>> the discussion going.
>>
>> Migration is irrelevant. The TSC page is guest global so updates will
>> happen on some (random) host CPU and therefor you need the usual barriers
>> like we have them in our seqcounts unless an access to the sequence will
>> trap into the host, which would defeat the whole purpose of the TSC page.
>>
>
> KY Srinivasan <kys@xxxxxxxxxxxxx> writes:
>
>> I checked with the folks on the Hyper-V side and they have confirmed that we need to
>> add memory barriers in the guest code to ensure the various reads from the TSC page are
>> correctly ordered - especially, the initial read of the sequence counter must have acquire
>> semantics. We should ensure that other reads from the TSC page are completed before the
>> second read of the sequence counter. I am working with the Windows team to correctly
>> reflect this algorithm in the Hyper-V specification.
>
>
> Thank you,
>
> do I get it right that combining the above I only need to replace
> virt_rmb() barriers with plain rmb() to get 'lfence' in hv_read_tsc_page
> (PATCH 2)? As members of struct ms_hyperv_tsc_page are volatile we don't
> need READ_ONCE(), compilers are not allowed to merge accesses. The
> resulting code looks good to me:

No, on multiple counts, unfortunately.

1. LFENCE is basically useless except for IO and for (Intel only)
rdtsc_ordered().  AFAIK there is literally no scenario under which
LFENCE is useful for access to normal memory.

2. The problem here has little to do with barriers.  You're doing:

read seq;
read var1;
read var2;
read tsc;
read seq again;

If the hypervisor updates things between reading var1 and var2 or
between reading var2 and tsc, you aren't guaranteed to notice unless
something fancy is going on regardless of barriers.  Similarly, if the
hypervisor starts updating, then you start and finish the whole
function, and then the hypervisor finishes, what guarantees you
notice?

This really needs a spec from the hypervisor folks as to what's going
on.  KVM does this horrible thing in which it sometimes freezes all
vCPUs when updating, for better or for worse.  Mostly for worse.  If
MS does the same thing, they should document it.

3. You need rdtsc_ordered().  Plain RDTSC is not ordered wrt other
memory access, and it can observably screw up as a result.

Also, Linux tries to avoid volatile variables, so please use READ_ONCE().

>
> (gdb) disassemble read_hv_clock_tsc
> Dump of assembler code for function read_hv_clock_tsc:
>    0xffffffff8102ca60 <+0>:     callq  0xffffffff816c7500 <__fentry__>
>    0xffffffff8102ca65 <+5>:     mov    0xf67974(%rip),%rcx        # 0xffffffff81f943e0 <tsc_pg>
>    0xffffffff8102ca6c <+12>:    jmp    0xffffffff8102ca87 <read_hv_clock_tsc+39>
>    0xffffffff8102ca6e <+14>:    lfence
>    0xffffffff8102ca71 <+17>:    mov    0x8(%rcx),%r9
>    0xffffffff8102ca75 <+21>:    mov    0x10(%rcx),%r8
>    0xffffffff8102ca79 <+25>:    nop
>    0xffffffff8102ca7a <+26>:    nop
>    0xffffffff8102ca7b <+27>:    nop
>    0xffffffff8102ca7c <+28>:    rdtsc
>    0xffffffff8102ca7e <+30>:    lfence
>    0xffffffff8102ca81 <+33>:    mov    (%rcx),%edi
>    0xffffffff8102ca83 <+35>:    cmp    %edi,%esi
>    0xffffffff8102ca85 <+37>:    je     0xffffffff8102caa3 <read_hv_clock_tsc+67>
>    0xffffffff8102ca87 <+39>:    mov    (%rcx),%esi
>    0xffffffff8102ca89 <+41>:    test   %esi,%esi
>    0xffffffff8102ca8b <+43>:    jne    0xffffffff8102ca6e <read_hv_clock_tsc+14>
>    0xffffffff8102ca8d <+45>:    push   %rbp
>    0xffffffff8102ca8e <+46>:    mov    $0x40000020,%edi
>    0xffffffff8102ca93 <+51>:    mov    %rsp,%rbp
>    0xffffffff8102ca96 <+54>:    and    $0xfffffffffffffff0,%rsp
>    0xffffffff8102ca9a <+58>:    callq  *0xffffffff81c36330
>    0xffffffff8102caa1 <+65>:    leaveq
>    0xffffffff8102caa2 <+66>:    retq
>    0xffffffff8102caa3 <+67>:    shl    $0x20,%rdx
>    0xffffffff8102caa7 <+71>:    or     %rdx,%rax
>    0xffffffff8102caaa <+74>:    mul    %r9
>    0xffffffff8102caad <+77>:    mov    %rdx,%rax
>    0xffffffff8102cab0 <+80>:    add    %r8,%rax
>    0xffffffff8102cab3 <+83>:    cmp    $0xffffffffffffffff,%rax
>    0xffffffff8102cab7 <+87>:    je     0xffffffff8102ca8d <read_hv_clock_tsc+45>
>    0xffffffff8102cab9 <+89>:    repz retq
> End of assembler dump.
>
> --
>   Vitaly



-- 
Andy Lutomirski
AMA Capital Management, LLC