Re: [Qemu-devel] [PATCH RFC 2/3] s390x/tcg: low-address protection support

[David Hildenbrand]

On 28.09.2017 06:50, Thomas Huth wrote:
> On 27.09.2017 19:00, David Hildenbrand wrote:
>> This is a neat way to implement low address protection, whereby
>> only the first 512 bytes of the first two pages (each 4096 bytes) of
>> every address space are protected.
>>
>> Store a tec of 0 for the access exception, this is what is defined by
>> Enhanced Suppression on Protection in case of a low address protection
>> (Bit 61 set to 0, rest undefined).
>>
>> We have to make sure to to pass the access address, not the masked page
>> address into mmu_translate*().
>>
>> Drop the check from testblock. So we can properly test this via
>> kvm-unit-tests.
>>
>> This will check every access going through one of the MMUs.
>>
>> Signed-off-by: David Hildenbrand <address@hidden>
>> ---
>>  target/s390x/excp_helper.c |  3 +-
>>  target/s390x/mem_helper.c  |  8 ----
>>  target/s390x/mmu_helper.c  | 96 
>> +++++++++++++++++++++++++++++-----------------
>>  3 files changed, 62 insertions(+), 45 deletions(-)
> [...]
>> diff --git a/target/s390x/mmu_helper.c b/target/s390x/mmu_helper.c
>> index 9daa0fd8e2..44a15449d2 100644
>> --- a/target/s390x/mmu_helper.c
>> +++ b/target/s390x/mmu_helper.c
>> @@ -106,6 +106,37 @@ static void trigger_page_fault(CPUS390XState *env, 
>> target_ulong vaddr,
>>      trigger_access_exception(env, type, ilen, tec);
>>  }
>>  
>> +/* check whether the address would be proteted by Low-Address Protection */
>> +static bool is_low_address(uint64_t addr)
>> +{
>> +    return addr < 512 || (addr >= 4096 && addr < 4607);
>> +}
> 
> I like the check from the kernel sources better:
> 
> static inline int is_low_address(unsigned long ga)
> {
>     /* Check for address ranges 0..511 and 4096..4607 */
>     return (ga & ~0x11fful) == 0;
> }
> 
> ... that might result in slightly faster code (depending on the
> compiler, of course).

I think that lim (readability) -> 0. Without that comment you're at
first sight really clueless what this is about.

My check exactly corresponds to the wording in the PoP (and smart
compilers should be able to optimize).

But I don't have a strong opinion on this micro optimization.

Thanks

> 
>  Thomas
> 


-- 

Thanks,

David