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Subject[PATCHv5 05/30] x86/tdx: Exclude shared bit from __PHYSICAL_MASK
Date
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@linux.intel.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
---
arch/x86/Kconfig | 1 +
arch/x86/coco/tdx.c | 8 ++++++++
2 files changed, 9 insertions(+)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 93e67842e369..d2f45e58e846 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -885,6 +885,7 @@ config INTEL_TDX_GUEST
depends on X86_64 && CPU_SUP_INTEL
depends on X86_X2APIC
select ARCH_HAS_CC_PLATFORM
+ 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/coco/tdx.c b/arch/x86/coco/tdx.c
index 912ef12e434e..34818dc31248 100644
--- a/arch/x86/coco/tdx.c
+++ b/arch/x86/coco/tdx.c
@@ -69,6 +69,14 @@ void __init tdx_early_init(void)

cc_set_vendor(CC_VENDOR_INTEL);

+ /*
+ * 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(gpa_width - 2, 0);
+
/*
* The highest bit of a guest physical address is the "sharing" bit.
* Set it for shared pages and clear it for private pages.
--
2.34.1
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