Re: [PATCH V3 01/19] x86: Create per-domain mapping of guest_root_pt

[Alejandro Vallejo <alejandro.vallejo@xxxxxxxxx>]

On 14/05/2024 09:03, Jan Beulich wrote:
> On 13.05.2024 17:27, Roger Pau Monné wrote:
>> On Mon, May 13, 2024 at 11:10:59AM +0000, Elias El Yandouzi wrote:
>>> @@ -317,6 +317,14 @@ extern unsigned long xen_phys_start;
>>>  #define ARG_XLAT_START(v)        \
>>>      (ARG_XLAT_VIRT_START + ((v)->vcpu_id << ARG_XLAT_VA_SHIFT))
>>>  
>>> +/* pv_root_pt mapping area. The fourth per-domain-mapping sub-area */
>>> +#define PV_ROOT_PT_MAPPING_VIRT_START   PERDOMAIN_VIRT_SLOT(3)
>>> +#define PV_ROOT_PT_MAPPING_ENTRIES      MAX_VIRT_CPUS
>>> +
>>> +/* The address of a particular VCPU's PV_ROOT_PT */
>>> +#define PV_ROOT_PT_MAPPING_VCPU_VIRT_START(v) \
>>> +    (PV_ROOT_PT_MAPPING_VIRT_START + ((v)->vcpu_id * PAGE_SIZE))
>>
>> I know we are not there yet, but I wonder if we need to start having
>> some non-shared per-cpu mapping area in the page-tables.  Right now
>> this is shared between all the vCPUs, as it's per-domain space
>> (instead of per-vCPU).
> 
> In turn requiring per-vCPU page tables, posing a problem when a guest
> uses the same page tables for multiple vCPU-s.
> 
> Jan
> 

True. Having separate page tables per CPU is an unavoidable end goal for
a hypervisor claiming to hold no secrets, however. Otherwise any CPU can
still speculatively read the stacks of other CPUs and take well-timed
glances over mappings set transiently by any other CPU.

Cheers,
Alejandro