[PATCH v2 1/5] x86/kexec: do unconditional WBINVD in stop_this_cpu()

[Kai Huang]

TL;DR:

Change to do unconditional WBINVD in stop_this_cpu() to cover kexec
support for both AMD SME and Intel TDX, despite there _was_ some issue
preventing doing so but now has got fixed.

Long version:

Both SME and TDX can leave caches in incoherent state due to memory
encryption.  During kexec, the caches must be flushed before jumping to
the second kernel to avoid silent memory corruption to the second kernel.

Currently, for SME the kernel only does WBINVD in stop_this_cpu() when
the kernel determines the hardware supports SME.  To support TDX, one
option is to extend that specific check to cover both SME and TDX.

However, instead of sprinkling around vendor-specific checks, it's
better to just do unconditional WBINVD.  Kexec() is a slow path, and it
is acceptable to have an additional WBINVD in order to have simple
and easy to maintain code.

Note:

Historically, there _was_ an issue preventing doing unconditional WBINVD
but that has been fixed.

When SME kexec() support was initially added in commit

  bba4ed011a52: ("x86/mm, kexec: Allow kexec to be used with SME")

WBINVD was done unconditionally.  However since then some issues were
reported that different Intel systems would hang or reset due to that
commit.

To try to fix, a later commit

  f23d74f6c66c: ("x86/mm: Rework wbinvd, hlt operation in stop_this_cpu()")

then changed to only do WBINVD when hardware supports SME.

While this commit made the reported issues go away, it didn't pinpoint
the root cause.  Also, it didn't handle a corner case[*] correctly, which
resulted in the reveal of the root cause and the final fix by commit

  1f5e7eb7868e: ("x86/smp: Make stop_other_cpus() more robust")

See [1][2] for more information.

Further testing of doing unconditional WBINVD based on the above fix on
the problematic machines (that issues were originally reported)
confirmed the issues couldn't be reproduced.

See [3][4] for more information.

Therefore, it is safe to do unconditional WBINVD now.

[*] The commit didn't check whether the CPUID leaf is available or not.
Making unsupported CPUID leaf on Intel returns garbage resulting in
unintended WBINVD which caused some issue (followed by the analysis and
the reveal of the final root cause).  The corner case was independently
fixed by commit

  9b040453d444: ("x86/smp: Dont access non-existing CPUID leaf")

[1]: https://lore.kernel.org/lkml/CALu+AoQKmeixJdkO07t7BtttN7v3RM4_aBKi642bQ3fTBbSAVg@xxxxxxxxxxxxxx/T/#m300f3f9790850b5daa20a71abcc200ae8d94a12a
[2]: https://lore.kernel.org/lkml/CALu+AoQKmeixJdkO07t7BtttN7v3RM4_aBKi642bQ3fTBbSAVg@xxxxxxxxxxxxxx/T/#ma7263a7765483db0dabdeef62a1110940e634846
[3]: https://lore.kernel.org/lkml/CALu+AoQKmeixJdkO07t7BtttN7v3RM4_aBKi642bQ3fTBbSAVg@xxxxxxxxxxxxxx/T/#mc043191f2ff860d649c8466775dc61ac1e0ae320
[4]: https://lore.kernel.org/lkml/CALu+AoQKmeixJdkO07t7BtttN7v3RM4_aBKi642bQ3fTBbSAVg@xxxxxxxxxxxxxx/T/#md23f1a8f6afcc59fa2b0ac1967f18e418e24347c

Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
Suggested-by: Borislav Petkov <bp@xxxxxxxxx>
Cc: Tom Lendacky <thomas.lendacky@xxxxxxx>
Cc: Dave Young <dyoung@xxxxxxxxxx>
---
 arch/x86/kernel/process.c | 17 ++++++-----------
 1 file changed, 6 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index b8441147eb5e..b375f069dd2a 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -813,19 +813,14 @@ void __noreturn stop_this_cpu(void *dummy)
 	mcheck_cpu_clear(c);
 
 	/*
-	 * Use wbinvd on processors that support SME. This provides support
-	 * for performing a successful kexec when going from SME inactive
-	 * to SME active (or vice-versa). The cache must be cleared so that
-	 * if there are entries with the same physical address, both with and
-	 * without the encryption bit, they don't race each other when flushed
-	 * and potentially end up with the wrong entry being committed to
-	 * memory.
+	 * The kernel could leave caches in incoherent state on SME/TDX
+	 * capable platforms.  Flush cache to avoid silent memory
+	 * corruption for these platforms.
 	 *
-	 * Test the CPUID bit directly because the machine might've cleared
-	 * X86_FEATURE_SME due to cmdline options.
+	 * stop_this_cpu() is not a fast path, just do unconditional
+	 * WBINVD for simplicity.
 	 */
-	if (c->extended_cpuid_level >= 0x8000001f && (cpuid_eax(0x8000001f) & BIT(0)))
-		native_wbinvd();
+	native_wbinvd();
 
 	/*
 	 * This brings a cache line back and dirties it, but
-- 
2.34.1