linux-kernel - Re: [PATCH v1 4/4] KVM: MMU: Expose the LA57 feature to VM.

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Date:   Fri, 18 Aug 2017 14:50:09 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Yu Zhang <yu.c.zhang@...ux.intel.com>, kvm@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, rkrcmar@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        xiaoguangrong@...cent.com, joro@...tes.org
Subject: Re: [PATCH v1 4/4] KVM: MMU: Expose the LA57 feature to VM.

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
>>
>