[PATCHv2 21/29] x86/tdx: Exclude shared bit from __PHYSICAL_MASK

[Kirill A. Shutemov]

In TDX guests, by default memory is protected from host access. If a
guest needs to communicate with the VMM (like the I/O use case), it uses
a single bit in the physical address to communicate the protected/shared
attribute of the given page.

In the x86 ARCH code, __PHYSICAL_MASK macro represents the width of the
physical address in the given architecture. It is used in creating
physical PAGE_MASK for address bits in the kernel. Since in TDX guest,
a single bit is used as metadata, it needs to be excluded from valid
physical address bits to avoid using incorrect addresses bits in the
kernel.

Enable DYNAMIC_PHYSICAL_MASK to support updating the __PHYSICAL_MASK.

Co-developed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 arch/x86/Kconfig      | 1 +
 arch/x86/kernel/tdx.c | 8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 1c59e02792e4..680c3cad9422 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -886,6 +886,7 @@ config INTEL_TDX_GUEST
 	depends on X86_X2APIC
 	select ARCH_HAS_CC_PLATFORM
 	select X86_MCE
+	select DYNAMIC_PHYSICAL_MASK
 	help
 	  Support running as a guest under Intel TDX.  Without this support,
 	  the guest kernel can not boot or run under TDX.
diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
index b27c4261bfd2..beeaf61934bc 100644
--- a/arch/x86/kernel/tdx.c
+++ b/arch/x86/kernel/tdx.c
@@ -486,5 +486,13 @@ void __init tdx_early_init(void)
 
 	tdx_get_info();
 
+	/*
+	 * All bits above GPA width are reserved and kernel treats shared bit
+	 * as flag, not as part of physical address.
+	 *
+	 * Adjust physical mask to only cover valid GPA bits.
+	 */
+	physical_mask &= GENMASK_ULL(td_info.gpa_width - 2, 0);
+
 	pr_info("Guest detected\n");
 }
-- 
2.34.1