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Subject: Re: [edk2-devel] [RFC PATCH 01/28] OvmfPkg/Sec: Enable cache early to speed up booting
To: devel@edk2.groups.io, thomas.lendacky@amd.com
Cc: Jordan Justen <jordan.l.justen@intel.com>,
 Ard Biesheuvel <ard.biesheuvel@linaro.org>,
 Michael D Kinney <michael.d.kinney@intel.com>,
 Liming Gao <liming.gao@intel.com>, Eric Dong <eric.dong@intel.com>,
 Ray Ni <ray.ni@intel.com>, "Singh, Brijesh" <brijesh.singh@amd.com>
References: <cover.1566250534.git.thomas.lendacky@amd.com>
 <6a37c84f4989304b21205d6263c6491f81da3233.1566250534.git.thomas.lendacky@amd.com>
From: "Laszlo Ersek" <lersek@redhat.com>
Message-ID: <6d3442d5-46ab-2b99-6100-0e5c56477735@redhat.com>
Date: Wed, 21 Aug 2019 16:21:25 +0200
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On 08/19/19 23:35, Lendacky, Thomas wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
> 
> Currently, the OVMF code relies on the hypervisor to enable the cache
> support on the processor in order to improve the boot speed. However,
> with SEV-ES, the hypervisor is not allowed to change the CR0 register
> to enable caching.
> 
> Update the OVMF Sec support to enable caching in order to improve the
> boot speed.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  OvmfPkg/Sec/SecMain.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/OvmfPkg/Sec/SecMain.c b/OvmfPkg/Sec/SecMain.c
> index 3914355cd17b..2448be0cd408 100644
> --- a/OvmfPkg/Sec/SecMain.c
> +++ b/OvmfPkg/Sec/SecMain.c
> @@ -739,6 +739,11 @@ SecCoreStartupWithStack (
>  
>    ProcessLibraryConstructorList (NULL, NULL);
>  
> +  //
> +  // Enable caching
> +  //
> +  AsmEnableCache ();
> +
>    DEBUG ((EFI_D_INFO,
>      "SecCoreStartupWithStack(0x%x, 0x%x)\n",
>      (UINT32)(UINTN)BootFv,
> 

This makes me uncomfortable. There used to be problems related to
caching when VFIO device assignment were used. My concern is admittedly
vague, but this is a very brittle area of OVMF-on-KVM. If you asked me
"well what could break here", I'd answer "you never know, and the burden
of proof is not on me". :) Can we make this change conditional on SEV-ES?

Thanks
Laszlo