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arch: x86_64: Stop applying SME c-bit reduction to phys_bits

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get_host_cpu_phys_bits() subtracts the SME c-bit reduction from
PhysAddrSize (CPUID 0x80000008 EAX bits 7:0). The result sets the
guest's CPUID and MMIO address space size.

The c-bit reduction is not needed here. QEMU's equivalent
(host_cpu_phys_bits() in target/i386/host-cpu.c) returns
PhysAddrSize without reduction.

Signed-off-by: Ruben Hakobyan <hruben@meta.com>
This commit is contained in:
Ruben Hakobyan
2026-05-28 19:01:32 -07:00
committed by Rob Bradford
parent eb75a4ead7
commit 99f5537984
1 changed files with 2 additions and 14 deletions
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16
arch/src/x86_64/mod.rs
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@@ -1413,27 +1413,15 @@ pub fn initramfs_load_addr(
Ok(aligned_addr)
}
pub fn get_host_cpu_phys_bits(hypervisor: &dyn hypervisor::Hypervisor) -> u8 {
pub fn get_host_cpu_phys_bits(_hypervisor: &dyn hypervisor::Hypervisor) -> u8 {
// SAFETY: call cpuid with valid leaves
#[allow(unused_unsafe)]
unsafe {
let leaf = x86_64::__cpuid(0x8000_0000);
// Detect and handle AMD SME (Secure Memory Encryption) properly.
// Some physical address bits may become reserved when the feature is enabled.
// See AMD64 Architecture Programmer's Manual Volume 2, Section 7.10.1
let reduced = if leaf.eax >= 0x8000_001f
&& matches!(hypervisor.get_cpu_vendor(), CpuVendor::AMD)
&& x86_64::__cpuid(0x8000_001f).eax & 0x1 != 0
{
(x86_64::__cpuid(0x8000_001f).ebx >> 6) & 0x3f
} else {
0
};
if leaf.eax >= 0x8000_0008 {
let leaf = x86_64::__cpuid(0x8000_0008);
((leaf.eax & 0xff) - reduced) as u8
(leaf.eax & 0xff) as u8
} else {
36
}