Re: [Xen-devel] [PATCH v3 24/35] OvmfPkg/XenPlatformPei: Rework memory detection

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Mon, Jul 15, 2019 at 04:15:21PM +0200, Roger Pau Monné wrote:
> On Thu, Jul 04, 2019 at 03:42:22PM +0100, Anthony PERARD wrote:
> > When running as a Xen PVH guest, there is no CMOS to read the memory
> > size from.  Rework GetSystemMemorySize(Below|Above)4gb() so they can
> > works without CMOS by reading the e820 table.
> > 
> > Rework XenPublishRamRegions for PVH, handle the Reserve type and explain
> > about the ACPI type. MTRR settings aren't modified anymore, on HVM, it's
> > already done by hvmloader, on PVH it is supposed to have sane default.
> > 
> > Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1689
> > Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> > Acked-by: Laszlo Ersek <lersek@xxxxxxxxxx>
> > ---
> > 
> > Notes:
> >     Comment for Xen people:
> >     About MTRR, should we redo the setting in OVMF? Even if in both case of
> >     PVH and HVM, something would have setup the default type to write back
> >     and handle a few other ranges like PCI hole, hvmloader for HVM or and
> >     libxc I think for PVH.
> 
> That's a tricky question. Ideally we would like the firmware (OVMF) to
> take care of that, because it already has code to do so. Problem here
> is that PVH can also be booted without firmware, in which case it
> needs the hypervisor to have setup some sane initial MTRR state.
> 
> The statement in the PVH document about initial MTRR state is vague
> enough that allows Xen to boot into the guest with a minimal MTRR
> state, that can for example not contain UC regions for the MMIO
> regions of passed through devices, hence I think OVMF should be in
> charge of creating a more complete MTRR state if possible.
> 
> Is this something OVMF already has logic for?

Well, there are some logic but it's for QEMU (and uses an interface that
isn't available when running on Xen, fwcfg).

The logic that was there for Xen HVM was very simple, a single set
cache-write-back for the RAM, that's why I remove it (and because I'm
not sure yet I figured out how to run the mtrr functions correctly in
OVMF).

I probably going to have to write a new logic which would rewrite the
MTRR from scratch instead of relying on the existing setup.

> Also accounting for the MMIO regions of devices?

I'll have to dig deeper into OVMF codes, and PCI device handling. On
HVM, we have a different logic than the one for QEMU, OVMF only scan
what hvmloader have done instead of re-setup the pci devices. I'm
probably missing other stuff.

> > diff --git a/OvmfPkg/XenPlatformPei/MemDetect.c 
> > b/OvmfPkg/XenPlatformPei/MemDetect.c
> > index cb7dd93ad6..3e33e7f414 100644
> > --- a/OvmfPkg/XenPlatformPei/MemDetect.c
> > +++ b/OvmfPkg/XenPlatformPei/MemDetect.c
> > @@ -96,6 +96,47 @@ Q35TsegMbytesInitialization (
> >    mQ35TsegMbytes = ExtendedTsegMbytes;
> >  }
> >  
> > +STATIC
> > +UINT64
> > +GetHighestSystemMemoryAddress (
> > +  BOOLEAN       Below4gb
> > +  )
> > +{
> > +  EFI_E820_ENTRY64    *E820Map;
> > +  UINT32              E820EntriesCount;
> > +  EFI_E820_ENTRY64    *Entry;
> > +  EFI_STATUS          Status;
> > +  UINT32              Loop;
> > +  UINT64              HighestAddress;
> > +  UINT64              EntryEnd;
> > +
> > +  HighestAddress = 0;
> > +
> > +  Status = XenGetE820Map (&E820Map, &E820EntriesCount);
> 
> You could maybe initialize this as a global to avoid having to issue
> a hypercall each time you need to get something from the memory map.

That function does that, it only make the hypercall once. (The hypercall
can only be made once anyway, the second time Xen doesn't return the
map.)

> > +  ASSERT_EFI_ERROR (Status);
> > +
> > +  for (Loop = 0; Loop < E820EntriesCount; Loop++) {
> > +    Entry = E820Map + Loop;
> > +    EntryEnd = Entry->BaseAddr + Entry->Length;
> > +
> > +    if (Entry->Type == EfiAcpiAddressRangeMemory &&
> > +        EntryEnd > HighestAddress) {
> > +
> > +      if (Below4gb && (EntryEnd <= BASE_4GB)) {
> > +        HighestAddress = EntryEnd;
> > +      } else if (!Below4gb && (EntryEnd >= BASE_4GB)) {
> > +        HighestAddress = EntryEnd;
> > +      }
> > +    }
> > +  }
> > +
> > +  //
> > +  // Round down the end address.
> > +  //
> > +  HighestAddress &= ~(UINT64)EFI_PAGE_MASK;
> > +
> > +  return HighestAddress;
> 
> You could do the rounding on the return statement.

Yes, I think that can be done.

> > +}
> >  
> >  UINT32
> >  GetSystemMemorySizeBelow4gb (
> > @@ -105,6 +146,19 @@ GetSystemMemorySizeBelow4gb (
> >    UINT8 Cmos0x34;
> >    UINT8 Cmos0x35;
> >  
> > +  //
> > +  // In PVH case, there is no CMOS, we have to calculate the memory size
> > +  // from parsing the E820
> > +  //
> > +  if (XenPvhDetected ()) {
> 
> IIRC on HVM you can also get the memory map from the hypercall, in
> which case you could use the same code path for both HVM and PVH.

I think that wouldn't work because in my experiment, the hypercall would
only return the map the first time (at least on PVH). hvmloader already
make the hypercall so OVMF can't.

On the other hand, XenGetE820Map() return an E820 map, it doesn't matter
if it's the one passed by hvmloader, or the one we've got directly from
Xen. So I guess we could ignore what hvmloader have written in the CMOS
and use the information in the e820 directly. But I think I let this
change for future patch.

> > +    UINT64  HighestAddress;
> > +
> > +    HighestAddress = GetHighestSystemMemoryAddress (TRUE);
> > +    ASSERT (HighestAddress > 0 && HighestAddress <= BASE_4GB);
> > +
> > +    return HighestAddress;
> 
> The name of the function here is GetSystemMemorySizeBelow4gb, but you
> are returning the highest memory address in the range, is this
> expected?
> 
> ie: highest address != memory size
> 
> On HVM there are quite some holes in the memory map, and nothing
> guarantees there are no memory regions after the holes or non-RAM
> regions.

I think that's what is expected by caller of the function.

> > +  }
> > +
> >    //
> >    // CMOS 0x34/0x35 specifies the system memory above 16 MB.
> >    // * CMOS(0x35) is the high byte
> > diff --git a/OvmfPkg/XenPlatformPei/Xen.c b/OvmfPkg/XenPlatformPei/Xen.c
> > index cbfd8058fc..62a2c3ed93 100644
> > --- a/OvmfPkg/XenPlatformPei/Xen.c
> > +++ b/OvmfPkg/XenPlatformPei/Xen.c
> > @@ -279,6 +279,8 @@ XenPublishRamRegions (
> >    EFI_E820_ENTRY64  *E820Map;
> >    UINT32            E820EntriesCount;
> >    EFI_STATUS        Status;
> > +  EFI_E820_ENTRY64 *Entry;
> > +  UINTN Index;
> >  
> >    DEBUG ((EFI_D_INFO, "Using memory map provided by Xen\n"));
> >  
> > @@ -287,26 +289,45 @@ XenPublishRamRegions (
> >    //
> >    E820EntriesCount = 0;
> >    Status = XenGetE820Map (&E820Map, &E820EntriesCount);
> > -
> >    ASSERT_EFI_ERROR (Status);
> >  
> > -  if (E820EntriesCount > 0) {
> > -    EFI_E820_ENTRY64 *Entry;
> > -    UINT32 Loop;
> > +  for (Index = 0; Index < E820EntriesCount; Index++) {
> > +    UINT64 Base;
> > +    UINT64 End;
> >  
> > -    for (Loop = 0; Loop < E820EntriesCount; Loop++) {
> > -      Entry = E820Map + Loop;
> > +    Entry = &E820Map[Index];
> >  
> > +
> > +    //
> > +    // Round up the start address, and round down the end address.
> > +    //
> > +    Base = ALIGN_VALUE (Entry->BaseAddr, (UINT64)EFI_PAGE_SIZE);
> > +    End = (Entry->BaseAddr + Entry->Length) & ~(UINT64)EFI_PAGE_MASK;
> > +
> > +    switch (Entry->Type) {
> > +    case EfiAcpiAddressRangeMemory:
> > +      AddMemoryRangeHob (Base, End);
> > +      break;
> > +    case EfiAcpiAddressRangeACPI:
> > +      //
> > +      // Ignore, OVMF should read the ACPI tables and provide them to linux
> > +      // from a different location.
> 
> Will OVMF also parse dynamic tables to check for references there?

I haven't looked at what OVMF does with the ACPI tables, but Linux seems
fine. I've compared the boot output of linux running as PVH vs booted
via OVMF. Beside the location of the table been different, the number of
table where the same, I don't remember other difference.

> > +      //
> > +      break;
> > +    case EfiAcpiAddressRangeReserved:
> >        //
> > -      // Only care about RAM
> > +      // Avoid ranges marked as reserved in the e820 table provided by
> > +      // hvmloader as it conflicts with an other aperture.
> 
> I think you want the last part of the sentence to be: '... as it
> conflicts with other apertures.'
> 
> I think however that you should make sure ranges marked as reserved in
> the original memory map also end up in the final one, hence
> overlapping ranges should be merged, instead of discarded.

I'll look into that.

> > +      // error message: CpuDxe: IntersectMemoryDescriptor:
> > +      //        desc [FC000000, 100000000) type 1 cap 8700000000026001
> > +      //        conflicts with aperture [FEE00000, FEE01000) cap 1
> >        //
> > -      if (Entry->Type != EfiAcpiAddressRangeMemory) {
> > -        continue;
> > +      if (!XenHvmloaderDetected ()) {
> > +        AddReservedMemoryBaseSizeHob (Base, End - Base, FALSE);
> 
> This special casing for PVH looks weird, ideally we would like to use
> the same code path, or else it should be explicitly mentioned why PVH
> has diverging behaviour.

I think hvmloader is the issue rather than PVH. Here is part of the
"memory map" as found in hvmloader/config.h:

  /* Special BIOS mappings, etc. are allocated from here upwards... */
  #define RESERVED_MEMBASE              0xFC000000
  /* NB. ACPI_INFO_PHYSICAL_ADDRESS *MUST* match definition in acpi/dsdt.asl! */
  #define ACPI_INFO_PHYSICAL_ADDRESS    0xFC000000
  #define RESERVED_MEMORY_DYNAMIC_START 0xFC001000
  #define RESERVED_MEMORY_DYNAMIC_END   0xFE000000

and hvmloader simply creates a single e820 reserved entry, from
RESERVED_MEMBASE to the top of 4GB. It's probably too much.
If hvmloader only reserved
ACPI_INFO_PHYSICAL_ADDRESS-RESERVED_MEMORY_DYNAMIC_END, I might not have
to special case hvmloader. As far as I know 0xfee00000 isn't a special
bios mapping, but something the hardware provides.


Anyway, thanks for the feedback, there's probably quite a bit to do to
cleanup the memory stuff. I do think about one day running OVMF without
running hvmloader first :-), but there's a bit more to do.

-- 
Anthony PERARD

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