Re: [PATCH v6 3/6] sev/i386: Allow AP booting under SEV-ES

[Tom Lendacky <thomas.lendacky@amd.com>]

On 1/29/21 11:44 AM, Venu Busireddy wrote:
> On 2021-01-26 11:36:46 -0600, Tom Lendacky wrote:
>> From: Tom Lendacky <thomas.lendacky@amd.com>
>>
>> When SEV-ES is enabled, it is not possible modify the guests register
>> state after it has been initially created, encrypted and measured.
>>
>> Normally, an INIT-SIPI-SIPI request is used to boot the AP. However, the
>> hypervisor cannot emulate this because it cannot update the AP register
>> state. For the very first boot by an AP, the reset vector CS segment
>> value and the EIP value must be programmed before the register has been
>> encrypted and measured. Search the guest firmware for the guest for a
>> specific GUID that tells Qemu the value of the reset vector to use.
>>
>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>> Cc: "Michael S. Tsirkin" <mst@redhat.com>
>> Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
>> Cc: Richard Henderson <richard.henderson@linaro.org>
>> Cc: Eduardo Habkost <ehabkost@redhat.com>
>> Cc: Marcelo Tosatti <mtosatti@redhat.com>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
...
>> +
>> +    /*
>> +     * SEV info block not found in the Firmware GUID Table (or there isn't
>> +     * a Firmware GUID Table), fall back to the original implementation.
>> +     */
>> +    data = flash_ptr + flash_size - 0x20;
> 
> Even if the SEV_INFO_BLOCK_GUID is always located at 32 bytes from the end
> of the flash, isn't it better to define a constant with a value of 0x20?

A follow-on patch that updates both this and the table parser code from 
James would probably be best.

Thanks,
Tom

> 
>> +