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

Ard Biesheuvel
2016-06-30T10:42:11Z

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.

> + __hyp_do_panic(str_va,
> spsr, elr,
> read_sysreg(esr_el2), read_sysreg_el2(far),
> read_sysreg(hpfar_el2), par,
>
> With that in place, we can entirely get rid of hyp_kern_va().
>
> M.
> --
> Jazz is not dead. It just smells funny...
>
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