Re: [edk2-devel] [RFC PATCH v3 33/43] MdeModulePkg: Reserve a 16-bit protected mode code segment descriptor

["Lendacky, Thomas" <thomas.lendacky@amd.com>]

On 12/12/19 12:57 AM, Ni, Ray via Groups.Io wrote:
> Is the 16bit code segment descriptor needed in early DXE before CpuDxe (DXE MP)?

I don't believe it is. I did this to keep all of the CS descriptors
consistent since the ones here seemed to line up with the ones in
UefiCpuPkg/CpuDxe/CpuGdt.c.  I can remove this change if that isn't
a requirement.

Thanks,
Tom

> 
>> -----Original Message-----
>> From: Tom Lendacky <thomas.lendacky@amd.com>
>> Sent: Thursday, November 21, 2019 4:07 AM
>> To: devel@edk2.groups.io
>> Cc: Justen, Jordan L <jordan.l.justen@intel.com>; Laszlo Ersek <lersek@redhat.com>; Ard Biesheuvel
>> <ard.biesheuvel@linaro.org>; Kinney, Michael D <michael.d.kinney@intel.com>; Gao, Liming <liming.gao@intel.com>; Dong,
>> Eric <eric.dong@intel.com>; Ni, Ray <ray.ni@intel.com>; Brijesh Singh <brijesh.singh@amd.com>; Wang, Jian J
>> <jian.j.wang@intel.com>; Wu, Hao A <hao.a.wu@intel.com>; Bi, Dandan <dandan.bi@intel.com>
>> Subject: [RFC PATCH v3 33/43] MdeModulePkg: Reserve a 16-bit protected mode code segment descriptor
>>
>> BZ: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.tianocore.org%2Fshow_bug.cgi%3Fid%3D2198&amp;data=02%7C01%7Cthomas.lendacky%40amd.com%7C8ec88e8912cd4b99eb0f08d77ed09b2c%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637117306738013792&amp;sdata=PwSQc53mtPMUTALaRhQrcQzGI3ooPUMhcktYLi1ezfk%3D&amp;reserved=0
>>
>> SEV-ES guest AP boot support needs to transition from 64-bit long mode
>> into 16-bit real mode. This will require a 16-bit code segment descriptor.
>> Reserve one of the spare segment descriptors (0x28) for this purpose.
>>
>> Cc: Jian J Wang <jian.j.wang@intel.com>
>> Cc: Hao A Wu <hao.a.wu@intel.com>
>> Cc: Dandan Bi <dandan.bi@intel.com>
>> Cc: Liming Gao <liming.gao@intel.com>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>>  MdeModulePkg/Core/DxeIplPeim/Ia32/DxeLoadFunc.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/MdeModulePkg/Core/DxeIplPeim/Ia32/DxeLoadFunc.c b/MdeModulePkg/Core/DxeIplPeim/Ia32/DxeLoadFunc.c
>> index 284b34818ca7..c9cf2e36214f 100644
>> --- a/MdeModulePkg/Core/DxeIplPeim/Ia32/DxeLoadFunc.c
>> +++ b/MdeModulePkg/Core/DxeIplPeim/Ia32/DxeLoadFunc.c
>> @@ -33,7 +33,7 @@ GLOBAL_REMOVE_IF_UNREFERENCED IA32_GDT gGdtEntries[] = {
>>  /* 0x10 */  {{0xffff, 0,  0,  0xf,  1,  0,  1,  0xf,  0,  0, 1,  1,  0}}, //linear code segment descriptor
>>  /* 0x18 */  {{0xffff, 0,  0,  0x3,  1,  0,  1,  0xf,  0,  0, 1,  1,  0}}, //system data segment descriptor
>>  /* 0x20 */  {{0xffff, 0,  0,  0xa,  1,  0,  1,  0xf,  0,  0, 1,  1,  0}}, //system code segment descriptor
>> -/* 0x28 */  {{0,      0,  0,  0,    0,  0,  0,  0,    0,  0, 0,  0,  0}}, //spare segment descriptor
>> +/* 0x28 */  {{0xffff, 0,  0,  0xa,  1,  0,  1,  0xf,  0,  0, 0,  1,  0}}, //system code16 segment descriptor
>>  /* 0x30 */  {{0xffff, 0,  0,  0x2,  1,  0,  1,  0xf,  0,  0, 1,  1,  0}}, //system data segment descriptor
>>  /* 0x38 */  {{0xffff, 0,  0,  0xa,  1,  0,  1,  0xf,  0,  1, 0,  1,  0}}, //system code segment descriptor
>>  /* 0x40 */  {{0,      0,  0,  0,    0,  0,  0,  0,    0,  0, 0,  0,  0}}, //spare segment descriptor
>> --
>> 2.17.1
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