Re: [PATCH] x86: Don't crash when mapping a page using EFI runtime page tables

[Ross Lagerwall <ross.lagerwall@citrix.com>]

On 05/27/2015 12:59 PM, Jan Beulich wrote:
>>>> On 27.05.15 at 12:23, <ross.lagerwall@citrix.com> wrote:
>> On 05/18/2015 03:58 PM, Jan Beulich wrote:
>>>>>> On 15.05.15 at 18:08, <ross.lagerwall@citrix.com> wrote:
>>>> --- a/xen/arch/x86/domain_page.c
>>>> +++ b/xen/arch/x86/domain_page.c
>>>> @@ -32,20 +32,25 @@ static inline struct vcpu *mapcache_current_vcpu(void)
>>>>            return NULL;
>>>>
>>>>        /*
>>>> +     * When using efi runtime page tables, we have the equivalent of the
>> idle
>>>> +     * domain's page tables but current may point at another domain's VCPU.
>>>> +     * Return NULL as though current is not properly set up yet.
>>>> +     */
>>>> +    if ( efi_enabled && read_cr3() == efi_rs_page_table() )
>>>> +        return NULL;
>>>
>>> I'm okay with the patch in principle; what worries me is the CR3 read
>>> that is now going to be necessary even in non-debug builds. With
>>> this code being the only user of efi_rs_page_table(), I wonder if it
>>> wouldn't make sense to alter that function to return non-zero only
>>> when spin_is_locked(&efi_rs_lock), and then alter the code above
>>> such that the CR3 read would happen only when we got a non-zero
>>> value back.
>>
>> mapcache_current_vcpu() appears to be called from IRQ-enabled and
>> IRQ-disabled callers which prevents us from using the spinlock.
>
> I didn't suggest to use any spin lock; I merely suggested checking
> whether that particular one is being held by someone (to avoid the
> CR3 read if that's not the case).

spin_is_locked() calls check_lock() which causes a BUG_ON() even though 
you're not actually using the lock.

-- 
Ross Lagerwall