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Re: [PATCH for-4.21 1/2] x86/AMD: avoid REP MOVSB for Zen3/4
- To: Teddy Astie <teddy.astie@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 25 Sep 2025 14:58:58 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 25 Sep 2025 12:59:06 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 25.09.2025 14:18, Teddy Astie wrote:
> Le 25/09/2025 à 12:48, Jan Beulich a écrit :
>> Along with Zen2 (which doesn't expose ERMS), both families reportedly
>> suffer from sub-optimal aliasing detection when deciding whether REP MOVSB
>> can actually be carried out the accelerated way. Therefore we want to
>> avoid its use in the common case (memset(), copy_page_hot()).
>
> s/memset/memcpy (memset probably uses rep stosb which is not affected IIUC)
Oops, yes.
>> Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> Question is whether merely avoiding REP MOVSB (but not REP MOVSQ) is going
>> to be good enough.
>
> This probably wants to be checked with benchmarks of rep movsb vs rep
> movsq+b (current non-ERMS algorithm). If the issue also occurs with rep
> movsq, it may be preferable to keep rep movsb even considering this issue.
Why? Then REP MOVSB is 8 times slower than REP MOVSQ.
>> --- a/xen/arch/x86/cpu/amd.c
>> +++ b/xen/arch/x86/cpu/amd.c
>> @@ -1386,6 +1386,10 @@ static void cf_check init_amd(struct cpu
>>
>> check_syscfg_dram_mod_en();
>>
>> + if (c == &boot_cpu_data && cpu_has(c, X86_FEATURE_ERMS)
>> + && c->family != 0x19 /* Zen3/4 */)
>> + setup_force_cpu_cap(X86_FEATURE_XEN_REP_MOVSB);
>> +
>
> May it be fixed through a (future ?) microcode update, especially since
> rep movs is microcoded on these archs ?
I don't know, but I also don't expect that to happen.
Jan
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