linux-kernel - Re: [PATCH Part2 RFC v4 01/40] KVM: SVM: Add support to handle AP reset MSR protocol

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Date:   Thu, 15 Jul 2021 09:39:58 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Brijesh Singh <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,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.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>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>, tony.luck@...el.com,
        npmccallum@...hat.com, brijesh.ksingh@...il.com
Subject: Re: [PATCH Part2 RFC v4 01/40] KVM: SVM: Add support to handle AP
 reset MSR protocol

On Wed, Jul 14, 2021 at 08:17:29PM +0000, Sean Christopherson wrote:
> On Wed, Jul 07, 2021, Brijesh Singh wrote:
> > From: Tom Lendacky <thomas.lendacky@....com>
> > 
> > Add support for AP Reset Hold being invoked using the GHCB MSR protocol,
> > available in version 2 of the GHCB specification.
> 
> Please provide a brief overview of the protocol, and why it's needed.  I assume
> it's to allow AP wakeup without a shared GHCB?

Yes, this is needed for SEV-ES kexec support to park APs without the
need for memory that will be owned by the new kernel when APs are woken
up.

You can have a look into my SEV-ES kexec/kdump patch-set for details:

	https://lore.kernel.org/lkml/20210705082443.14721-1-joro@8bytes.org/

I also sent this patch separatly earlier this week to enable GHCB
protocol version 2 support in KVM.

Regards,

	Joerg