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

[Cornelia Huck <cohuck@redhat.com>]

On Thu, 28 Sep 2017 15:08:11 +0200
David Hildenbrand <david@redhat.com> wrote:

> 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 <david@redhat.com>
> >> ---
> >>  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.

FWIW, I'd be happy with both, but has anyone actually looked at the
generated code?