[RFC] Extending MTRRs above 4G

[Alex Williamson <alex.williamson@hp.com>]

When I try to boot guests using a recent Linux kernel (2.6.26+), memory
above 3.5G gets thrown away with an error like this:

WARNING: BIOS bug: CPU MTRRs don't cover all of memory, losing 4608MB of RAM.

And it's true, we're only providing MTRRs for memory below 4G.  In fact
rombios32 knows very little, if anything, about memory above 4G, as seen
by memory reporting in the SMBIOS table.

It looks like the Linux kernel MTRR code does have a bail-out point for
kvm/qemu, but that was only effective before we started reporting MTRRs.
On real hardware, I have two systems that do this two different ways.
The first is an Intel based system, which reports MTRRs to cover the I/O
space, then defaults the rest of memory to WB.  The second is an AMD
based system which uses MTRRs to cover memory below 4G, then seems to
have a special AMD MSR to describe the top of memory above 4G.  Xen
appears to mimic the first approach.

Is there any reason that KVM sets the default MTRR type to UC, then only
sets up MTRRs for the memory below 4G?  the patch below is a possible
approach to continue down this path and enlighten rombios32 about the
real top of memory, and setup MTRRs appropriately.  It doesn't address
SMBIOS or whatever causes grub to only report upper memory below 4G.
Alternatively we could switch to the Intel/Xen system approach, but it
seems rombios32 needs to understand the extra memory at some point
anyway.  Thoughts?  BTW, another benefit to the default WB approach is
that MTRRs are a limited resource and there will be memory sizes we
can't fully cover using the approach below.  Thanks,

Alex


Signed-off-by: Alex Williamson <alex.williamson@hp.com>
--

diff --git a/bios/rombios32.c b/bios/rombios32.c
index 2dc1d25..c57e967 100755
--- a/bios/rombios32.c
+++ b/bios/rombios32.c
@@ -416,6 +416,7 @@ uint32_t cpuid_signature;
 uint32_t cpuid_features;
 uint32_t cpuid_ext_features;
 unsigned long ram_size;
+uint64_t above4g_ram_size;
 uint8_t bios_uuid[16];
 #ifdef BX_USE_EBDA_TABLES
 unsigned long ebda_cur_addr;
@@ -530,6 +531,14 @@ void setup_mtrr(void)
         wrmsr_smp(MTRRphysMask_MSR(i), (~vmask & 0xfffffff000ull) | 0x800);
         vbase += vmask + 1;
     }
+    for (vbase = 1ull << 32; i < vcnt && vbase < above4g_ram_size; ++i) {
+        vmask = (1ull << 40) - 1;
+        while (vbase + vmask + 1 > above4g_ram_size)
+            vmask >>= 1;
+        wrmsr_smp(MTRRphysBase_MSR(i), vbase | 6);
+        wrmsr_smp(MTRRphysMask_MSR(i), (~vmask & 0xfffffff000ull) | 0x800);
+        vbase += vmask + 1;
+    }
     wrmsr_smp(MSR_MTRRdefType, 0xc00);
 }
 
@@ -540,10 +549,19 @@ void ram_probe(void)
         16 * 1024 * 1024;
   else
     ram_size = (cmos_readb(0x17) | (cmos_readb(0x18) << 8)) * 1024;
+
+  if (cmos_readb(0x5b) | cmos_readb(0x5c) | cmos_readb(0x5d))
+    above4g_ram_size = ((uint64_t)cmos_readb(0x5b) << 16) |
+        ((uint64_t)cmos_readb(0x5c) << 24) | ((uint64_t)cmos_readb(0x5d) << 32);
+
+  if (above4g_ram_size)
+    above4g_ram_size += 1ull << 32;
+
 #ifdef BX_USE_EBDA_TABLES
     ebda_cur_addr = ((*(uint16_t *)(0x40e)) << 4) + 0x380;
 #endif
     BX_INFO("ram_size=0x%08lx\n", ram_size);
+    BX_INFO("top of ram %ldMB\n", above4g_ram_size >> 20);
   setup_mtrr();
 }