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References: <20210913181941.23405-1-brijesh.singh@amd.com> <20210913181941.23405-10-brijesh.singh@amd.com>
In-Reply-To: <20210913181941.23405-10-brijesh.singh@amd.com>
From: "Erdem Aktas" <erdemaktas@google.com>
Date: Wed, 15 Sep 2021 20:08:19 +0300
Message-ID: <CAAYXXYyfo7REYVweW2R16cQgcD96d2S7ThNGEEcjSauPDAs2Qg@mail.gmail.com>
Subject: Re: [PATCH v7 09/31] OvmfPkg/SecMain: register GHCB gpa for the SEV-SNP guest
To: Brijesh Singh <brijesh.singh@amd.com>
Cc: edk2-devel-groups-io <devel@edk2.groups.io>, James Bottomley <jejb@linux.ibm.com>, 
	Min Xu <min.m.xu@intel.com>, Jiewen Yao <jiewen.yao@intel.com>, 
	Tom Lendacky <thomas.lendacky@amd.com>, Jordan Justen <jordan.l.justen@intel.com>, 
	Ard Biesheuvel <ardb+tianocore@kernel.org>, Michael Roth <Michael.Roth@amd.com>, 
	Gerd Hoffmann <kraxel@redhat.com>
Content-Type: text/plain; charset="UTF-8"

On Mon, Sep 13, 2021 at 9:20 PM Brijesh Singh <brijesh.singh@amd.com> wrote:
> +*/
> +STATIC
> +VOID
> +SevSnpGhcbRegister (
> +  UINTN   Address
> +  )
> +{
> +  MSR_SEV_ES_GHCB_REGISTER  Msr;
> +  MSR_SEV_ES_GHCB_REGISTER  CurrentMsr;
> +  EFI_PHYSICAL_ADDRESS      GuestFrameNumber;
> +
> +  GuestFrameNumber = Address >> EFI_PAGE_SHIFT;
> +
> +  //
> +  // Save the current MSR Value
> +  //
> +  CurrentMsr.GhcbPhysicalAddress = AsmReadMsr64 (MSR_SEV_ES_GHCB);

We are backing the current MSR value but when was it initialized
before ? Also is not this function supposed to set the Address as the
GHCB address? If it is, do we care about the old value?


> +  // Restore the MSR
> +  //
> +  AsmWriteMsr64 (MSR_SEV_ES_GHCB, CurrentMsr.GhcbPhysicalAddress);

Why are we restoring the old value? I may have misunderstood but I
thought this function will set Address as the new GHCB address?

Thanks
-Erdem