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SubjectRe: [PATCH v1 4/4] KVM: MMU: Expose the LA57 feature to VM.
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On 18/08/2017 10:28, Yu Zhang wrote:
>
>
> On 8/17/2017 10:29 PM, Paolo Bonzini wrote:
>> On 17/08/2017 13:53, Yu Zhang wrote:
>>>
>>> On 8/17/2017 7:57 PM, Paolo Bonzini wrote:
>>>> On 12/08/2017 15:35, Yu Zhang wrote:
>>>>> index a98b88a..50107ae 100644
>>>>> --- a/arch/x86/kvm/emulate.c
>>>>> +++ b/arch/x86/kvm/emulate.c
>>>>> @@ -694,7 +694,7 @@ static __always_inline int __linearize(struct
>>>>> x86_emulate_ctxt *ctxt,
>>>>> switch (mode) {
>>>>> case X86EMUL_MODE_PROT64:
>>>>> *linear = la;
>>>>> - if (is_noncanonical_address(la))
>>>>> + if (emul_is_noncanonical_address(la, ctxt))
>>>>> goto bad;
>>>>> *max_size = min_t(u64, ~0u, (1ull << 48) - la);
>>>> Oops, you missed one here. Probably best to use ctxt_virt_addr_bits
>>>> and
>>>> then "inline" emul_is_noncanonical_address as "get_canonical(la,
>>>> va_bits) != la".
>>> Sorry, I just sent out the v2 patch set without noticing this reply. :-)
>>>
>>> The emul_is_noncanonical() is defined in x86.h so that no
>>> ctxt_virt_addr_bits needed in emulate.c, are you
>>> suggesting to use ctx_virt_addr_bits in this file each time before
>>> emul_is_noncanonical_address() is called?
>> No, only in this instance which uses "48" after the call to
>> emul_is_noncanonical_address.
>
> Sorry, Paolo. I still do not quite get it.
> Do you mean the
> *max_size = min_t(u64, ~0u, (1ull << 48) - la);
> also need to be changed?
>
> But I do not understand why this statement is used like this. My
> understanding is that
> for 64 bit scenario, the *max_size is calculated to guarantee la +
> *max_size still falls in
> the canonical address space.
>
> And if above understanding is correct, I think it should be something
> like below:
> *max_size = min_t(u64, ~0u - la, (1ull << 48) - la);

The "~0u" part is simply because max_size has 32-bit size (it's an
unsigned int variable), while (1ull << 48) - la has 64-bit size. It
protects from the overflow.

> And with LA57, may better be changed to:
> *max_size = min_t(u64, ~0u - la, (1ull << ctxt_virt_addr_bits(ctxt)) -
> la);
>
> And for the above
> if (emul_is_noncanonical_address(la, ctxt))
> we may just leave it as it is.

Yes, exactly. But since emul_is_noncanonical_address is already using
ctxt_virt_addr_bits(ctxt), it may make sense to compute
ctxt_virt_addr_bits(ctxt) once and then reuse it twice, once in
get_canonical(la, va_bits) != la and once in (1ull << va_bits) - la.

Paolo

> Is this understanding correct? Or did I misunderstand your comments? :-)
>
> Thanks
> Yu
>> Paolo
>>
>

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