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

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Tue, Jul 23, 2019 at 11:42:07AM +0200, Roger Pau Monné wrote:
> On Mon, Jul 22, 2019 at 03:53:19PM +0100, Anthony PERARD wrote:
> > 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:
> > > 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.)
> 
> Why? I'm looking at the implementation in Xen of XENMEM_memory_map and
> I'm not sure I see how/why the hypercall can only be made once. AFAICT
> you should be able to call XENMEM_memory_map multiple times without
> issues, or else it's a bug somewhere.

:-(, I probably made a mistake when testing that. I tried again and
calling the hypercall serveral time gave the same result. Sorry for the
noise.

> > > > +}
> > > >  
> > > >  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.
> 
> OK, I'm not sure the reason for this, as I said above I think this is
> a bug somewhere. You should be able to call XENMEM_memory_map multiple
> times.
> 
> > 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
> 
> Hm, I'm not sure hvmloader uploads a new memory map to Xen (using
> XENMEM_set_memory_map) if it does any modifications to it. It should
> certainly do it, so that the guest OS gets the same memory map from
> the hypercall or from the firmware.

hvmloader doesn't call XENMEM_set_memory_map (I don't find that string
in the source code), also, I've tested again calling the get memory_map
hypercall in HVM guests and the e820 from hvmloader is different from
the one from the hypercall:

from hvmloader:
    Type Mem  - 00000000 -> 000A0000
    Type Res  - 000F0000 -> 00100000
    Type Mem  - 00100000 -> 3F6B3000
    Type Res  - FC000000 -> 100000000
from Xen:
    Type Mem  - 00100000 -> 3F800000

> > > > +    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.
> 
> OK, what I find weird is that you seem to discard quite a lot of stuff
> from the original memory map, and then reconstruct it afterwards I
> assume?
> 
> It would seem safer to not discard regions from the memory map
> provided to OVMF, and instead just build on top of it. I would expect

OK, I'll add back the EfiAcpiAddressRangeACPI into the reserved regions.

> for example that OVMF will use some of the RAM regions on the memory
> map, and it should likely turn those areas from RAM into reserved
> regions.
> 
> > > > +      // 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.
> 
> But isn't this kind of dangerous? How can you assure future versions
> of hvmloader won't use this space?
> 
> > If hvmloader only reserved
> > ACPI_INFO_PHYSICAL_ADDRESS-RESERVED_MEMORY_DYNAMIC_END, I might not have
> > to special case hvmloader.
> 
> Could we look into getting this fixed in hvmloader then?
> 
> I think it's dangerous for OVMF to play such tricks with the memory
> map.

It's not worse than before :), there's a lot more to cleanup :(.

I've left an hvmloader specific call
    AddReservedMemoryBaseSizeHob (0xFC000000, 0x1000000, FALSE);
which is outside of that XenPublishRamRegions() function. See:
https://lists.xenproject.org/archives/html/xen-devel/2019-07/msg00360.html
I probably need to remove that now and see if I can simply use
hvmloader's e820.

-- 
Anthony PERARD

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