linux-kernel - Re: [PATCH Part1 v5 07/38] x86/sev: Add support for hypervisor feature VMGEXIT

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Date:   Mon, 23 Aug 2021 13:34:49 -0500
From:   Brijesh Singh <brijesh.singh@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Andi Kleen <ak@...ux.intel.com>, tony.luck@...el.com,
        marcorr@...gle.com, sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH Part1 v5 07/38] x86/sev: Add support for hypervisor
 feature VMGEXIT



On 8/23/21 1:25 PM, Brijesh Singh wrote:
> 
> 
> On 8/23/21 4:47 AM, Borislav Petkov wrote:
>> On Fri, Aug 20, 2021 at 10:19:02AM -0500, Brijesh Singh wrote:
>>> Version 2 of GHCB specification introduced advertisement of a features
>>> that are supported by the hypervisor. Add support to query the HV
>>> features on boot.
>>>
>>> Version 2 of GHCB specification adds several new NAEs, most of them are
>>> optional except the hypervisor feature. Now that hypervisor feature NAE
>>> is implemented, so bump the GHCB maximum support protocol version.
>>>
>>> Signed-off-by: Brijesh Singh <brijesh.singh@....com>
>>> ---
>>>   arch/x86/include/asm/mem_encrypt.h |  2 ++
>>>   arch/x86/include/asm/sev-common.h  |  3 +++
>>>   arch/x86/include/asm/sev.h         |  2 +-
>>>   arch/x86/include/uapi/asm/svm.h    |  2 ++
>>>   arch/x86/kernel/sev-shared.c       | 23 +++++++++++++++++++++++
>>>   5 files changed, 31 insertions(+), 1 deletion(-)
>>
>> I think you can simplify more.
>>
>> The HV features are read twice - once in the decompressor stub and again
>> in kernel proper - but I guess that's not such a big deal.
>>
>> Also, sev_hv_features can be static.
>>
>> Diff ontop:
>>
> 
> The sev_hv_features is also referred during the AP creation. By caching 
> the value in sev-shared.c and exporting it to others, we wanted to 
> minimize VMGEXITs during the AP creation.
> 
> If we go with your patch below, then we will need to cache the 
> sev_hv_features in sev.c, so that it can be later used by the AP 
> creation code (see patch#22).
> 

Let me take it back, I didn't realize that sev.c includes the 
sev-shared.c. So your patch will work fine. sorry about the noise.