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Re: [PATCH v3 06/12] x86/shadow: restrict OOS allocation to when it's really needed
- To: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 23 Feb 2026 09:16:35 +0100
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- Cc: Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 23 Feb 2026 08:16:43 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 23.02.2026 09:11, Jan Beulich wrote:
> On 20.02.2026 16:06, Andrew Cooper wrote:
>> On 16/05/2023 8:40 am, Jan Beulich wrote:
>>> --- a/xen/arch/x86/mm/shadow/common.c
>>> +++ b/xen/arch/x86/mm/shadow/common.c
>>> @@ -61,7 +61,9 @@ const uint8_t sh_type_to_size[] = {
>>> [SH_type_l4_64_shadow] = 1,
>>> [SH_type_p2m_table] = 1,
>>> [SH_type_monitor_table] = 1,
>>> +#if (SHADOW_OPTIMIZATIONS & SHOPT_OUT_OF_SYNC)
>>> [SH_type_oos_snapshot] = 1,
>>> +#endif
>>> };
>>> #endif /* CONFIG_HVM */
>>>
>>> @@ -1771,7 +1773,8 @@ static void sh_update_paging_modes(struc
>>> #endif /* (SHADOW_OPTIMIZATIONS & SHOPT_VIRTUAL_TLB) */
>>>
>>> #if (SHADOW_OPTIMIZATIONS & SHOPT_OUT_OF_SYNC)
>>> - if ( mfn_eq(v->arch.paging.shadow.oos_snapshot[0], INVALID_MFN) )
>>> + if ( !(d->options & XEN_DOMCTL_CDF_oos_off) &&
>>> + mfn_eq(v->arch.paging.shadow.oos_snapshot[0], INVALID_MFN) )
>>> {
>>> int i;
>>
>> I've never seen XEN_DOMCTL_CDF_oos_off used. Xapi has no plumbing for
>> this, and xl only inherited it from xend.
>>
>> At this point, OOS is the tested path and OOS_OFF is the untested path.
>> I think we should remove the flag and let OOS be unconditional like all
>> the other shadow optimisations.
>
> It being a guest config setting, I'm hesitant. Maybe we should actually make
> sure it's tested, so that in the case it's needed it could be named as a
> mitigation for an XSA?
Also, ftaod, this change had gone in already long ago, with Roger's ack.
Jan
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