Re: [PATCH v12 05/14] x86/sgx: Implement basic EPC misc cgroup functionality

Huang, Kai
2024-04-18T23:30:32Z

> Was requested by Jarkko:
> https://lore.kernel.org/lkml/CYU504RLY7QU.QZY9LWC076NX@suppilovahvero/#t
>
>> [...]

Ah I missed that. No problem to me.

>>
>>>
>>> --- /dev/null
>>> +++ b/arch/x86/kernel/cpu/sgx/epc_cgroup.h
>>> @@ -0,0 +1,72 @@
>>> +/* SPDX-License-Identifier: GPL-2.0 */
>>> +#ifndef _SGX_EPC_CGROUP_H_
>>> +#define _SGX_EPC_CGROUP_H_
>>> +
>>> +#include <asm/sgx.h>
>>
>> I don't see why you need <asm/sgx.h> here.  Also, ...
>>
>>> +#include <linux/cgroup.h>
>>> +#include <linux/misc_cgroup.h>
>>> +
>>> +#include "sgx.h"
>>
>> ... "sgx.h" already includes <asm/sgx.h>
>>
>> [...]
>>
> right
>
>>>
>>> +static inline struct sgx_cgroup *sgx_get_current_cg(void)
>>> +{
>>> +    /* get_current_misc_cg() never returns NULL when Kconfig enabled */
>>> +    return sgx_cgroup_from_misc_cg(get_current_misc_cg());
>>> +}
>>
>> I spent some time looking into this.  And yes if I was reading code
>> correctly the get_current_misc_cg() should never return NULL when Kconfig
>> is on.
>>
>> I typed my analysis below in [*].  And it would be helpful if any cgroup
>> expert can have a second eye on this.
>>
>> [...]
>>
> Thanks for checking this and I did similar and agree with the
> conclusion. I think this is confirmed also by Michal's description AFAICT:
> "
> The current implementation creates root css object (see cgroup_init(),
> cgroup_ssid_enabled() check is after cgroup_init_subsys()).
> I.e. it will look like all tasks are members of root cgroup wrt given
> controller permanently and controller attribute files won't exist."

After looking I believe we can even disable MISC cgroup at runtime for a
particular cgroup (haven't actually verified on real machine, though):

# echo "-misc" > /sys/fs/cgroup/my_group/cgroup.subtree_control

And if you look at the MISC cgroup core code, many functions actually
handle a NULL css, e.g., misc_cg_try_charge():

int misc_cg_try_charge(enum misc_res_type type,
struct misc_cg *cg, u64 amount)
{
...

if (!(valid_type(type) && cg &&
READ_ONCE(misc_res_capacity[type])))
return -EINVAL;

...
}

That's why I am still a little bit worried about this. And it's better
to have cgroup expert(s) to confirm here.

Btw, AMD SEV doesn't need to worry because it doesn't dereference @css
but just pass it to MISC cgroup core functions like
misc_cg_try_charge(). But for SGX, we actually dereference it directly.