[PATCH v4] x86/asm: Force native_apic_mem_read() to use the MOV instruction

[Adam Dunlap]

When done from a virtual machine, instructions that touch APIC memory
must be emulated. By convention, MMIO access are typically performed via
io.h helpers such as 'readl()' or 'writeq()' to simplify instruction
emulation/decoding (ex: in KVM hosts and SEV guests) [0].

Currently, native_apic_mem_read() does not follow this convention,
allowing the compiler to emit instructions other than the MOV
instruction generated by readl(). In particular, when compiled with
clang and run as a SEV-ES or SEV-SNP guest, the compiler would emit a
TESTL instruction which is not supported by the SEV-ES emulator, causing
a boot failure in that environment. It is likely the same problem would
happen in a TDX guest as that uses the same instruction emulator as
SEV-ES.

To make sure all emulators can emulate APIC memory reads via MOV, use
the readl() function in native_apic_mem_read(). It is expected that any
emulator would support MOV in any addressing mode it is the most generic
and is what is ususally emitted currently.

The TESTL instruction is emitted when native_apic_mem_read() is inlined
into apic_mem_wait_icr_idle(). The emulator comes from insn_decode_mmio
in arch/x86/lib/insn-eval.c. It's not worth it to extend
insn_decode_mmio to support more instructions since, in theory, the
compiler could choose to output nearly any instruction for such reads
which would bloat the emulator beyond reason.

[0] https://lore.kernel.org/all/20220405232939.73860-12-kirill.shutemov@xxxxxxxxxxxxxxx/

Signed-off-by: Adam Dunlap <acdunlap@xxxxxxxxxx>
Tested-by: Kevin Loughlin <kevinloughlin@xxxxxxxxxx>
Reviewed-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx

---

An alterative to this approach would be to use inline assembly instead
of the readl() helper, as that is what native_apic_mem_write() does. I
consider using readl() to be cleaner since it is documented to be a simple
wrapper and inline assembly is less readable. native_apic_mem_write()
cannot be trivially updated to use writel since it appears to use custom
asm to workaround for a processor-specific bug.

Patch changelog:
V1 -> V2: Replaced asm with readl function which does the same thing
V2 -> V3: Updated commit message to show more motivation and
justification
V3 -> V4: Fixed nits in commit message

Link to v2 discussion: https://lore.kernel.org/all/20220908170456.3177635-1-acdunlap@xxxxxxxxxx/

 arch/x86/include/asm/apic.h | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index 9d159b771dc8..dddd3fc195ef 100644
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -13,6 +13,7 @@
 #include <asm/mpspec.h>
 #include <asm/msr.h>
 #include <asm/hardirq.h>
+#include <asm/io.h>
 
 #define ARCH_APICTIMER_STOPS_ON_C3	1
 
@@ -96,7 +97,7 @@ static inline void native_apic_mem_write(u32 reg, u32 v)
 
 static inline u32 native_apic_mem_read(u32 reg)
 {
-	return *((volatile u32 *)(APIC_BASE + reg));
+	return readl((void __iomem *)(APIC_BASE + reg));
 }
 
 static inline void native_apic_mem_eoi(void)
-- 
2.43.0.594.gd9cf4e227d-goog