linux-kernel - Re: [PATCH v8 11/40] x86/sev: Register GHCB memory when SEV-SNP is active

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Message-ID: <7cedae5f-96e1-cb37-1ed1-6f8e7a23868b@amd.com>
Date:   Wed, 22 Dec 2021 09:16:54 -0600
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>,
        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>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v8 11/40] x86/sev: Register GHCB memory when SEV-SNP is
 active



On 12/22/21 7:16 AM, Borislav Petkov wrote:
> On Fri, Dec 10, 2021 at 09:43:03AM -0600, Brijesh Singh wrote:
>> @@ -652,7 +652,7 @@ static enum es_result vc_handle_msr(struct ghcb *ghcb, struct es_em_ctxt *ctxt)
>>    * This function runs on the first #VC exception after the kernel
>>    * switched to virtual addresses.
>>    */
>> -static bool __init sev_es_setup_ghcb(void)
>> +static bool __init setup_ghcb(void)
>>   {
>>   	/* First make sure the hypervisor talks a supported protocol. */
>>   	if (!sev_es_negotiate_protocol())
> 
> Ok, let me stare at this for a while:
> 
> This gets called by handle_vc_boot_ghcb() which gets set at build time:
> 
> arch/x86/kernel/head_64.S:372:SYM_DATA(initial_vc_handler,      .quad handle_vc_boot_ghcb)
> 
> initial_vc_handler() gets called by vc_boot_ghcb() which gets set in
> 
> early_setup_idt()
> 
> and that function already does sev_snp_register_ghcb().
> 
> So why don't you concentrate the work setup_ghcb() does before the first
> #VC and call it in early_setup_idt(), before the IDT is set?
> 
> And then you get rid of yet another setup-at-first-use case?
> 

I was following the existing SEV-ES implementation in which GHCB is 
setup on first #VC. But recently you recommended to move the setup 
outside of the VC handler for the decompression path and I was going to 
do the same for the kernel proper. I have tried moving the GHCB setup 
outside and it seems to be working okay with me (a limited testing so 
far). I will check Jorge to see if there was any reason for doing the 
GHCB setup inside the VC for the SEV-ES case.