Re: [PATCH v3 69/75] x86/realmode: Setup AP jump table

Tom Lendacky
2020-05-29T16:21:38Z

On 5/29/20 4:02 AM, Borislav Petkov wrote:
> On Tue, Apr 28, 2020 at 05:17:19PM +0200, Joerg Roedel wrote:
>> From: Tom Lendacky <thomas.lendacky@amd.com>
>>
>> Setup the AP jump table to point to the SEV-ES trampoline code so that
>> the APs can boot.
>
> Tom, in his laconic way, doesn't want to explain to us why is this even
> needed...
>
> :)

Looks like some of the detail was lost during the patch shuffling.
Originally (on GitHub) this was the text:

As part of the GHCB specification, the booting of APs under SEV-ES
requires an AP jump table when transitioning from one layer of code to
another (e.g. when going from UEFI to the OS). As a result, each layer
that parks an AP must provide the physical address of an AP jump table
to the next layer using the GHCB MSR.

Upon booting of the kernel, read the GHCB MSR and save the address of
the AP jump table. Under SEV-ES, APs are started using the INIT-SIPI-SIPI
sequence. Before issuing the first SIPI request for an AP, the start eip
is programmed into the AP jump table. Upon issuing the SIPI request, the
AP will awaken and jump to the start eip address programmed into the AP
jump table.

It needs to change "GHCB MSR" to "VMGEXIT MSR protocol", but should cover
what you're looking for.

Thanks,
Tom

>
> /me reads the code
>
> /me reads the GHCB spec
>
> aha, it gets it from the HV. And it can be set by the guest too...
>
> So how about expanding that commit message as to why this is done, why
> needed, etc?
>
> Thx.
>
>> diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
>> index 262f83cad355..1c5cbfd102d5 100644
>> --- a/arch/x86/realmode/init.c
>> +++ b/arch/x86/realmode/init.c
>> @@ -9,6 +9,7 @@
>> #include <asm/realmode.h>
>> #include <asm/tlbflush.h>
>> #include <asm/crash.h>
>> +#include <asm/sev-es.h>
>>
>> struct real_mode_header *real_mode_header;
>> u32 *trampoline_cr4_features;
>> @@ -107,6 +108,11 @@ static void __init setup_real_mode(void)
>> if (sme_active())
>> trampoline_header->flags |= TH_FLAGS_SME_ACTIVE;
>>
>> + if (sev_es_active()) {
>> + if (sev_es_setup_ap_jump_table(real_mode_header))
>> + panic("Failed to update SEV-ES AP Jump Table");
>> + }
>> +
>
> So this function gets slowly sprinkled with
>
> if (sev-something)
> bla
>
> Please wrap at least those last two into a
>
> sev_setup_real_mode()
>
> or so.
>