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

Ard Biesheuvel
2016-06-30T11:10:44Z

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 ...