Re: [edk2-devel] [PATCH V1 1/1] UefiCpuPkg/ResetVector: Cache Disable should not be set by default in CR0

["Min Xu" <min.m.xu@intel.com>]

Add intel linux guys in CC list.

> -----Original Message-----
> From: Gerd Hoffmann <kraxel@redhat.com>
> Sent: Wednesday, January 24, 2024 12:12 AM
> To: Laszlo Ersek <lersek@redhat.com>
> Cc: devel@edk2.groups.io; Johnson, Brian <brian.johnson@hpe.com>; West,
> Catharine <catharine.west@intel.com>; Xu, Min M <min.m.xu@intel.com>; Ni,
> Ray <ray.ni@intel.com>; Wu, MingliangX <mingliangx.wu@intel.com>; Yao,
> Jiewen <jiewen.yao@intel.com>; Xue, Shengfeng
> <xueshengfeng@byosoft.com.cn>; Dong, Eric <eric.dong@intel.com>; Kumar,
> Rahul R <rahul.r.kumar@intel.com>; De, Debkumar
> <debkumar.de@intel.com>
> Subject: Re: Re: [edk2-devel] [PATCH V1 1/1] UefiCpuPkg/ResetVector: Cache
> Disable should not be set by default in CR0
> 
>   Hi,
> 
> > >>> Well, it's OVMF in a virtual machine.  No boot guard involved.
> > >>> So we could probably go for a OVMF-specific patch here.
> > >>>
> > >>> But I'd prefer to figure what exactly is happening here before
> > >>> going down that route.  An extreme slowdown just because we flip
> > >>> that bit doesn't make sense to me.
> > >>>
> > >>>> Why is boot time increasing?
> > >>>
> > >>> Not clear.  It seems to be the lzma uncompress of the firmware
> > >>> volume in rom / pflash which is very slow.  Also it is apparently
> > >>> only triggered in case pci device assignment is used.
> > >>
> > >> I've seen extreme slowness on physical platforms when we've mixed
> > >> up the MTRRs or page tables, causing code to be mapped uncached.
> > >>
> > >> Lzma uncompress of ROM could be pretty slow as well, if the ROM is
> > >> being read uncached.  Lzma probably reads the data a byte at a
> > >> time, which is the worst case for uncached accesses.  Since this is
> > >> a VM, it's not actually uncached at the hardware level, but I don't
> > >> know how QEMU/KVM handles uncached guest mappings.... It may be
> doing a VMEXIT for every byte.
> > >>
> > >> Anyway, I suggest double-checking your page tables and MTRRs.
> > >
> > > It happens very early at boot, before MTRRs are setup, running on
> > > the initial page tables created by the OVMF reset vector.  The
> > > initial page tables have just 'accessed', 'dirty', 'read/write' and
> > > 'present' bits set for the 0-4G identity mapping.
> > >
> > > It seems to have something to do with EPT.  It does not happen on
> > > AMD processors.  It also does not happen when disabling EPT support
> > > in kvm on the host machine.
> > >
> > > looked at kvm kernel traces, I don't see excessive vmexits.
> >
> > This discussion evokes vague memories in me. I'll dump them here, but
> > I have no idea if they will be useful. (They probably won't.)
> >
> > - edk2 commit 98f378a7be12 ("OvmfPkg/ResetVector: enable caching in
> > initial page tables", 2013-09-24)
> >
> > - Linux (host) commit 879ae1880449 ("KVM: x86: obey
> > KVM_X86_QUIRK_CD_NW_CLEARED in kvm_set_cr0()", 2015-11-04)
> 
> I actually waded through the source code in both places ;)
> 
> Turned out kvm propagates guest MTRR settings to EPT memory types, but
> only in case kvm_arch_has_noncoherent_dma() is true, which why this
> triggers only with a mdev device assigned.
> 
> MTRR disabled gets translated to UNCACHABLE, this is where the slowdown
> comes from.  Test patch below fixes it for me.
> 
> take care,
>   Gerd
> 
> ----------------------------- cut here ---------------------------- commit
> eb9f40ffd8afad03ac1fb6ac0e2a9af12ae78152
> Author: Gerd Hoffmann <kraxel@redhat.com>
> Date:   Tue Jan 23 15:33:51 2024 +0100
> 
>     OvmfPkg/Sec: early mtrr setup
> 
>     Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> 
> diff --git a/OvmfPkg/Library/PlatformInitLib/MemDetect.c
> b/OvmfPkg/Library/PlatformInitLib/MemDetect.c
> index f042517bb64a..14f39236e44d 100644
> --- a/OvmfPkg/Library/PlatformInitLib/MemDetect.c
> +++ b/OvmfPkg/Library/PlatformInitLib/MemDetect.c
> @@ -1081,12 +1081,14 @@ PlatformQemuInitializeRam (
>    if (IsMtrrSupported () && (PlatformInfoHob->HostBridgeDevId !=
> CLOUDHV_DEVICE_ID)) {
>      MtrrGetAllMtrrs (&MtrrSettings);
> 
> +#if 0
>      //
>      // MTRRs disabled, fixed MTRRs disabled, default type is uncached
>      //
>      ASSERT ((MtrrSettings.MtrrDefType & BIT11) == 0);
>      ASSERT ((MtrrSettings.MtrrDefType & BIT10) == 0);
>      ASSERT ((MtrrSettings.MtrrDefType & 0xFF) == 0);
> +#endif
> 
>      //
>      // flip default type to writeback
> diff --git a/OvmfPkg/Sec/SecMain.c b/OvmfPkg/Sec/SecMain.c index
> 9bd1b9c95227..2820be1bab7c 100644
> --- a/OvmfPkg/Sec/SecMain.c
> +++ b/OvmfPkg/Sec/SecMain.c
> @@ -30,6 +30,7 @@
>  #include <Ppi/MpInitLibDep.h>
>  #include <Library/TdxHelperLib.h>
>  #include <Library/CcProbeLib.h>
> +#include <Register/Intel/ArchitecturalMsr.h>
>  #include "AmdSev.h"
> 
>  #define SEC_IDT_ENTRY_COUNT  34
> @@ -956,6 +957,19 @@ SecCoreStartupWithStack (
>    InitializeApicTimer (0, MAX_UINT32, TRUE, 5);
>    DisableApicTimerInterrupt ();
> 
> +  //
> +  // Early MTRR setup (enable + set sefault)  //  {
> +    MSR_IA32_MTRR_DEF_TYPE_REGISTER  DefType;
> +
> +    DefType.Uint64    = 0;
> +    DefType.Bits.Type = 6; /* write back */
> +    DefType.Bits.E    = 1; /* enable */
> +    AsmWriteMsr64 (MSR_IA32_MTRR_DEF_TYPE, DefType.Uint64);
> +    DEBUG ((DEBUG_ERROR, "%a:%d early mtrr: %lx\n", __func__, __LINE__,
> + DefType.Uint64));  }
> +
>    //
>    // Initialize Debug Agent to support source level debug in SEC/PEI phases
> before memory ready.
>    //



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