Re: [edk2-devel] [PATCH v4 32/40] OvmfPkg/Sec: Add #VC exception handling for Sec phase

["Laszlo Ersek" <lersek@redhat.com>]

On 02/05/20 00:01, Lendacky, Thomas wrote:
> BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2198
> 
> An SEV-ES guest will generate a #VC exception when it encounters a
> non-automatic exit (NAE) event. It is expected that the #VC exception
> handler will communicate with the hypervisor using the GHCB to handle
> the NAE event.
> 
> NAE events can occur during the Sec phase, so initialize exception
> handling early in the OVMF Sec support.
> 
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> ---
>  OvmfPkg/Sec/SecMain.inf |  2 ++
>  OvmfPkg/Sec/SecMain.c   | 76 ++++++++++++++++++++++++++++++++++-------
>  2 files changed, 65 insertions(+), 13 deletions(-)
> 
> diff --git a/OvmfPkg/Sec/SecMain.inf b/OvmfPkg/Sec/SecMain.inf
> index 63ba4cb555fb..401d06039dd3 100644
> --- a/OvmfPkg/Sec/SecMain.inf
> +++ b/OvmfPkg/Sec/SecMain.inf
> @@ -50,11 +50,13 @@ [LibraryClasses]
>    PeCoffExtraActionLib
>    ExtractGuidedSectionLib
>    LocalApicLib
> +  CpuExceptionHandlerLib
>  
>  [Ppis]
>    gEfiTemporaryRamSupportPpiGuid                # PPI ALWAYS_PRODUCED
>  
>  [Pcd]
> +  gUefiCpuPkgTokenSpaceGuid.PcdSevEsWorkAreaBase
>    gUefiOvmfPkgTokenSpaceGuid.PcdOvmfPeiMemFvBase
>    gUefiOvmfPkgTokenSpaceGuid.PcdOvmfPeiMemFvSize
>    gUefiOvmfPkgTokenSpaceGuid.PcdOvmfDxeMemFvBase
> diff --git a/OvmfPkg/Sec/SecMain.c b/OvmfPkg/Sec/SecMain.c
> index bae9764577f0..2bab7128ade2 100644
> --- a/OvmfPkg/Sec/SecMain.c
> +++ b/OvmfPkg/Sec/SecMain.c
> @@ -24,6 +24,7 @@
>  #include <Library/PeCoffExtraActionLib.h>
>  #include <Library/ExtractGuidedSectionLib.h>
>  #include <Library/LocalApicLib.h>
> +#include <Library/CpuExceptionHandlerLib.h>
>  
>  #include <Ppi/TemporaryRamSupport.h>
>  
> @@ -34,6 +35,10 @@ typedef struct _SEC_IDT_TABLE {
>    IA32_IDT_GATE_DESCRIPTOR  IdtTable[SEC_IDT_ENTRY_COUNT];
>  } SEC_IDT_TABLE;
>  
> +typedef struct _SEC_SEV_ES_WORK_AREA {
> +  UINT8  SevEsEnabled;
> +} SEC_SEV_ES_WORK_AREA;
> +
>  VOID
>  EFIAPI
>  SecStartupPhase2 (
> @@ -712,6 +717,19 @@ FindAndReportEntryPoints (
>    return;
>  }
>  
> +STATIC
> +BOOLEAN
> +SevEsIsEnabled (
> +  VOID
> +  )
> +{
> +  SEC_SEV_ES_WORK_AREA  *SevEsWorkArea;
> +
> +  SevEsWorkArea = (SEC_SEV_ES_WORK_AREA *) FixedPcdGet32 (PcdSevEsWorkAreaBase);
> +
> +  return ((SevEsWorkArea != NULL) && (SevEsWorkArea->SevEsEnabled != 0));
> +}
> +

Ah so this is how we handle the IA32 build then -- the PCD will be zero,
except in "OvmfPkgX64.fdf". OK.

>  VOID
>  EFIAPI
>  SecCoreStartupWithStack (
> @@ -737,8 +755,53 @@ SecCoreStartupWithStack (
>      Table[Index] = 0;
>    }
>  
> +  //
> +  // Initialize IDT - Since this is before library constructors are called,
> +  // we use a loop rather than CopyMem.
> +  //
> +  IdtTableInStack.PeiService = NULL;
> +  for (Index = 0; Index < SEC_IDT_ENTRY_COUNT; Index ++) {
> +    UINT8  *Src, *Dst;
> +    UINTN  Byte;
> +
> +    Src = (UINT8 *) &mIdtEntryTemplate;
> +    Dst = (UINT8 *) &IdtTableInStack.IdtTable[Index];
> +    for (Byte = 0; Byte < sizeof (mIdtEntryTemplate); Byte++) {
> +      Dst[Byte] = Src[Byte];
> +    }
> +  }
> +
> +  IdtDescriptor.Base  = (UINTN)&IdtTableInStack.IdtTable;
> +  IdtDescriptor.Limit = (UINT16)(sizeof (IdtTableInStack.IdtTable) - 1);
> +
> +  if (SevEsIsEnabled()) {
> +    //
> +    // For SEV-ES guests, the exception handler is needed before calling
> +    // ProcessLibraryConstructorList() because some of the library constructors
> +    // perform some functions that result in #VC exceptions being generated.
> +    //
> +    // Due to this code executing before library constructors, *all* library
> +    // API calls are theoretically interface contract violations. However,
> +    // because this is SEC (executing in flash), those constructors cannot
> +    // write variables with static storage duration anyway. Furthermore, only
> +    // a small, restricted set of APIs, such as AsmWriteIdtr() and
> +    // InitializeCpuExceptionHandlers(), are called, where we require that the
> +    // underlying library not require constructors to have been invoked and
> +    // that the library instance not trigger any #VC exceptions.
> +    //
> +    AsmWriteIdtr (&IdtDescriptor);
> +    InitializeCpuExceptionHandlers (NULL);
> +  }
> +
>    ProcessLibraryConstructorList (NULL, NULL);
>  
> +  if (!SevEsIsEnabled()) {
> +    //
> +    // For non SEV-ES guests, just load the IDTR.
> +    //
> +    AsmWriteIdtr (&IdtDescriptor);
> +  }
> +
>    DEBUG ((EFI_D_INFO,
>      "SecCoreStartupWithStack(0x%x, 0x%x)\n",
>      (UINT32)(UINTN)BootFv,
> @@ -751,19 +814,6 @@ SecCoreStartupWithStack (
>    //
>    InitializeFloatingPointUnits ();
>  
> -  //
> -  // Initialize IDT
> -  //
> -  IdtTableInStack.PeiService = NULL;
> -  for (Index = 0; Index < SEC_IDT_ENTRY_COUNT; Index ++) {
> -    CopyMem (&IdtTableInStack.IdtTable[Index], &mIdtEntryTemplate, sizeof (mIdtEntryTemplate));
> -  }
> -
> -  IdtDescriptor.Base  = (UINTN)&IdtTableInStack.IdtTable;
> -  IdtDescriptor.Limit = (UINT16)(sizeof (IdtTableInStack.IdtTable) - 1);
> -
> -  AsmWriteIdtr (&IdtDescriptor);
> -
>  #if defined (MDE_CPU_X64)
>    //
>    // ASSERT that the Page Tables were set by the reset vector code to
> 

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

Thanks,
Laszlo