Re: [PATCH 03/13] x86/HV: Add new hvcall guest address host visibility support

[Tianyu Lan]

On 7/29/2021 1:06 AM, Dave Hansen wrote:
> On 7/28/21 7:52 AM, Tianyu Lan wrote:
> > @@ -1986,7 +1988,9 @@ static int __set_memory_enc_dec(unsigned long addr, int numpages, bool enc)
> >   	int ret;
> >   
> >   	/* Nothing to do if memory encryption is not active */
> > -	if (!mem_encrypt_active())
> > +	if (hv_is_isolation_supported())
> > +		return hv_set_mem_enc(addr, numpages, enc);
> > +	else if (!mem_encrypt_active())
> >   		return 0;
>
> One more thing.  If you're going to be patching generic code, please
> start using feature checks that can get optimized away at runtime.
> hv_is_isolation_supported() doesn't look like the world's cheapest
> check.  It can't be inlined and costs at least a function call.

Yes, you are right. How about adding a static branch key for the check 
of isolation VM? This may reduce the check cost.