Re: [PATCH 05/15] arm64: KVM: Refactor kern_hyp_va/hyp_kern_va to deal with multiple offsets

Marc Zyngier
2016-06-30T11:57:42Z

On 30/06/16 12:10, Ard Biesheuvel wrote:
> On 30 June 2016 at 13:02, Marc Zyngier <marc.zyngier@arm.com> wrote:
>> On 30/06/16 11:42, Ard Biesheuvel wrote:
>>> On 30 June 2016 at 12:16, Marc Zyngier <marc.zyngier@arm.com> wrote:
>>>> On 30/06/16 10:22, Marc Zyngier wrote:
>>>>> On 28/06/16 13:42, Christoffer Dall wrote:
>>>>>> On Tue, Jun 07, 2016 at 11:58:25AM +0100, Marc Zyngier wrote:
>>>>>>> As we move towards a selectable HYP VA range, it is obvious that
>>>>>>> we don't want to test a variable to find out if we need to use
>>>>>>> the bottom VA range, the top VA range, or use the address as is
>>>>>>> (for VHE).
>>>>>>>
>>>>>>> Instead, we can expand our current helpers to generate the right
>>>>>>> mask or nop with code patching. We default to using the top VA
>>>>>>> space, with alternatives to switch to the bottom one or to nop
>>>>>>> out the instructions.
>>>>>>>
>>>>>>> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
>>>>>>> ---
>>>>>>> arch/arm64/include/asm/kvm_hyp.h | 27 ++++++++++++--------------
>>>>>>> arch/arm64/include/asm/kvm_mmu.h | 42 +++++++++++++++++++++++++++++++++++++---
>>>>>>> 2 files changed, 51 insertions(+), 18 deletions(-)
>>>>>>>
>>>>>>> diff --git a/arch/arm64/include/asm/kvm_hyp.h b/arch/arm64/include/asm/kvm_hyp.h
>>>>>>> index 61d01a9..dd4904b 100644
>>>>>>> --- a/arch/arm64/include/asm/kvm_hyp.h
>>>>>>> +++ b/arch/arm64/include/asm/kvm_hyp.h
>>>>>>> @@ -25,24 +25,21 @@
>>>>>>>
>>>>>>> #define __hyp_text __section(.hyp.text) notrace
>>>>>>>
>>>>>>> -static inline unsigned long __kern_hyp_va(unsigned long v)
>>>>>>> -{
>>>>>>> - asm volatile(ALTERNATIVE("and %0, %0, %1",
>>>>>>> - "nop",
>>>>>>> - ARM64_HAS_VIRT_HOST_EXTN)
>>>>>>> - : "+r" (v) : "i" (HYP_PAGE_OFFSET_MASK));
>>>>>>> - return v;
>>>>>>> -}
>>>>>>> -
>>>>>>> -#define kern_hyp_va(v) (typeof(v))(__kern_hyp_va((unsigned long)(v)))
>>>>>>> -
>>>>>>> static inline unsigned long __hyp_kern_va(unsigned long v)
>>>>>>> {
>>>>>>> - asm volatile(ALTERNATIVE("orr %0, %0, %1",
>>>>>>> - "nop",
>>>>>>> + u64 mask;
>>>>>>> +
>>>>>>> + asm volatile(ALTERNATIVE("mov %0, %1",
>>>>>>> + "mov %0, %2",
>>>>>>> + ARM64_HYP_OFFSET_LOW)
>>>>>>> + : "=r" (mask)
>>>>>>> + : "i" (~HYP_PAGE_OFFSET_HIGH_MASK),
>>>>>>> + "i" (~HYP_PAGE_OFFSET_LOW_MASK));
>>>>>>> + asm volatile(ALTERNATIVE("nop",
>>>>>>> + "mov %0, xzr",
>>>>>>> ARM64_HAS_VIRT_HOST_EXTN)
>>>>>>> - : "+r" (v) : "i" (~HYP_PAGE_OFFSET_MASK));
>>>>>>> - return v;
>>>>>>> + : "+r" (mask));
>>>>>>> + return v | mask;
>>>>>>
>>>>>> If mask is ~HYP_PAGE_OFFSET_LOW_MASK how can you be sure that setting
>>>>>> bit (VA_BITS - 1) is always the right thing to do to generate a kernel
>>>>>> address?
>>>>>
>>>>> It has taken be a while, but I think I finally see what you mean. We
>>>>> have no idea whether that bit was set or not.
>>>>>
>>>>>> This is kind of what I asked before only now there's an extra bit not
>>>>>> guaranteed by the architecture to be set for the kernel range, I
>>>>>> think.
>>>>>
>>>>> Yeah, I finally connected the couple of neurons left up there (that's
>>>>> what remains after the whole brexit braindamage). This doesn't work (or
>>>>> rather it only works sometimes). The good new is that I also realized we
>>>>> don't need any of that crap.
>>>>>
>>>>> The only case we currently use a HVA->KVA transformation is to pass the
>>>>> panic string down to panic(), and we can perfectly prevent
>>>>> __kvm_hyp_teardown from ever be evaluated as a HVA with a bit of
>>>>> asm-foo. This allows us to get rid of this whole function.
>>>>
>>>> Here's what I meant by this:
>>>>
>>>> diff --git a/arch/arm64/kvm/hyp/switch.c b/arch/arm64/kvm/hyp/switch.c
>>>> index 437cfad..c19754d 100644
>>>> --- a/arch/arm64/kvm/hyp/switch.c
>>>> +++ b/arch/arm64/kvm/hyp/switch.c
>>>> @@ -299,9 +299,16 @@ static const char __hyp_panic_string[] = "HYP panic:\nPS:%08llx PC:%016llx ESR:%
>>>>
>>>> static void __hyp_text __hyp_call_panic_nvhe(u64 spsr, u64 elr, u64 par)
>>>> {
>>>> - unsigned long str_va = (unsigned long)__hyp_panic_string;
>>>> + unsigned long str_va;
>>>>
>>>> - __hyp_do_panic(hyp_kern_va(str_va),
>>>> + /*
>>>> + * Force the panic string to be loaded from the literal pool,
>>>> + * making sure it is a kernel address and not a PC-relative
>>>> + * reference.
>>>> + */
>>>> + asm volatile("ldr %0, =__hyp_panic_string" : "=r" (str_va));
>>>> +
>>>
>>> Wouldn't it suffice to make __hyp_panic_string a non-static pointer
>>> to const char? That way, it will be statically initialized with a
>>> kernel VA, and the external linkage forces the compiler to evaluate
>>> its value at runtime.
>>
>> Yup, that would work as well. The only nit is that the pointer needs to be
>> in the __hyp_text section, and my compiler is shouting at me with this:
>>
>> CC arch/arm64/kvm/hyp/switch.o
>> arch/arm64/kvm/hyp/switch.c: In function '__hyp_call_panic_vhe':
>> arch/arm64/kvm/hyp/switch.c:298:13: error: __hyp_panic_string causes a section type conflict with __fpsimd_enabled_nvhe
>> const char *__hyp_panic_string __section(.hyp.text) = "HYP panic:\nPS:%08llx PC:%016llx ESR:%08llx\nFAR:%016llx HPFAR:%016llx PAR:%016llx\nVCPU:%p\n";
>> ^
>> arch/arm64/kvm/hyp/switch.c:22:24: note: '__fpsimd_enabled_nvhe' was declared here
>> static bool __hyp_text __fpsimd_enabled_nvhe(void)
>>
>> Any clue?
>>
>
> The pointer is writable/non-exec and the code is readonly/exec, so it
> makes sense for the compiler to complain about this. It needs to be
> non-const, though, to prevent the compiler from short-circuiting the
> evaluation, so the only solution would be to add a .hyp.data section
> to the linker script, and put the __hyp_panic_string pointer in there.
>
> Not worth the trouble, perhaps ...

Yeah. Slightly overkill for something that is not meant to be used...
I'll keep the asm hack for now, with a mental note of moving this to a
.hyp.data section if we ever create one for other reasons.

Thanks,

M.
--
Jazz is not dead. It just smells funny...