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

Marc Zyngier
2016-06-30T11:02:07Z

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?

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