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

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Message-ID: <ec1d4cfd-bbbc-e27a-7589-e85d9f0438f4@intel.com>
Date:   Thu, 29 Jul 2021 07:09:55 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Tianyu Lan <ltykernel@...il.com>, kys@...rosoft.com,
        haiyangz@...rosoft.com, sthemmin@...rosoft.com, wei.liu@...nel.org,
        decui@...rosoft.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, peterz@...radead.org,
        konrad.wilk@...cle.com, boris.ostrovsky@...cle.com,
        jgross@...e.com, sstabellini@...nel.org, joro@...tes.org,
        will@...nel.org, davem@...emloft.net, kuba@...nel.org,
        jejb@...ux.ibm.com, martin.petersen@...cle.com, arnd@...db.de,
        hch@....de, m.szyprowski@...sung.com, robin.murphy@....com,
        thomas.lendacky@....com, brijesh.singh@....com, ardb@...nel.org,
        Tianyu.Lan@...rosoft.com, rientjes@...gle.com,
        martin.b.radev@...il.com, akpm@...ux-foundation.org,
        rppt@...nel.org, kirill.shutemov@...ux.intel.com,
        aneesh.kumar@...ux.ibm.com, krish.sadhukhan@...cle.com,
        saravanand@...com, xen-devel@...ts.xenproject.org,
        pgonda@...gle.com, david@...hat.com, keescook@...omium.org,
        hannes@...xchg.org, sfr@...b.auug.org.au,
        michael.h.kelley@...rosoft.com
Cc:     iommu@...ts.linux-foundation.org, linux-arch@...r.kernel.org,
        linux-hyperv@...r.kernel.org, linux-kernel@...r.kernel.org,
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        vkuznets@...hat.com, anparri@...rosoft.com
Subject: Re: [PATCH 03/13] x86/HV: Add new hvcall guest address host
 visibility support

On 7/29/21 6:01 AM, Tianyu Lan wrote:
> 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.

I don't think you need a static key.

There are basically three choices:
1. Use an existing X86_FEATURE bit.  I think there's already one for
   when you are running under a hypervisor.  It's not super precise,
   but it's better than what you have.
2. Define a new X86_FEATURE bit for when you are running under
   Hyper-V.
3. Define a new X86_FEATURE bit specifically for Hyper-V isolation VM
   support.  This particular feature might be a little uncommon to
   deserve its own bit.

I'd probably just do #2.