Re: [Xen-devel] [PATCH 4/9] x86/HVM: move NOFLUSH handling out of hvm_set_cr3()

[Jan Beulich <jbeulich@suse.com>]

On 12.09.2019 13:35, Roger Pau Monné  wrote:
> On Wed, Sep 11, 2019 at 05:23:20PM +0200, Jan Beulich wrote:
>> The bit is meaningful only for MOV-to-CR3 insns, not anywhere else, in
>> particular not when loading nested guest state.
> 
> Can't you use the current vcpu to check if the guest is in nested
> mode, and avoid having to explicitly pass the noflush parameter?

Even if this implication held today (it doesn't according to
the uses in hvmemul_write_cr() and hvm_mov_to_cr()), I don't
think introducing such a dependency would be a good idea.

>> @@ -2282,12 +2287,11 @@ int hvm_set_cr0(unsigned long value, boo
>>      return X86EMUL_OKAY;
>>  }
>>  
>> -int hvm_set_cr3(unsigned long value, bool may_defer)
>> +int hvm_set_cr3(unsigned long value, bool noflush, bool may_defer)
> 
> I would rather introduce a flush parameter instead, I think negated
> booleans are harder to parse mentally, and easier to get wrong.

I did actually consider this, but decided against for the
reason of this "no flush" behavior being a later addition to
the effects CR3 writes have, i.e. I'd intentionally like it
to be in line with X86_CR3_NOFLUSH.

Jan

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