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SubjectRe: [PATCH v5 08/22] x86/virt/tdx: Shut down TDX module in case of error
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On Mon, 2022-06-27 at 13:46 -0700, Dave Hansen wrote:
> On 6/26/22 22:26, Kai Huang wrote:
> > On Fri, 2022-06-24 at 11:50 -0700, Dave Hansen wrote:
> > > So, the last patch was called:
> > >
> > > Implement SEAMCALL function
> > >
> > > and yet, in this patch, we have a "seamcall()" function. That's a bit
> > > confusing and not covered at *all* in this subject.
> > >
> > > Further, seamcall() is the *ONLY* caller of __seamcall() that I see in
> > > this series. That makes its presence here even more odd.
> > >
> > > The seamcall() bits should either be in their own patch, or mashed in
> > > with __seamcall().
> >
> > Right. The reason I didn't put the seamcall() into previous patch was it is
> > only used in this tdx.c, so it should be static. But adding a static function
> > w/o using it in previous patch will trigger a compile warning. So I introduced
> > here where it is first used.
> >
> > One option is I can introduce seamcall() as a static inline function in tdx.h in
> > previous patch so there won't be a warning. I'll change to use this way.
> > Please let me know if you have any comments.
>
> Does a temporary __unused get rid of the warning?

Yes, and both __maybe_unused and __always_unused also git rid of the warning
too.

__unused is not defined in compiler_attributes.h, so we need to use
__attribute__((__unused__)) explicitly, or have __unused defined to it as a
macro.

I think I can just use __always_unused for this purpose?

So I think we put seamcall() implementation to the patch which implements
__seamcall(). And we can inline for seamcall() and put it in either tdx.h or
tdx.c, or we can use __always_unused (or the one you prefer) to get rid of the
warning.

What's your opinion?
>
> > > > /*
> > > > * Detect and initialize the TDX module.
> > > > *
> > > > @@ -138,7 +195,10 @@ static int init_tdx_module(void)
> > > >
> > > > static void shutdown_tdx_module(void)
> > > > {
> > > > - /* TODO: Shut down the TDX module */
> > > > + struct seamcall_ctx sc = { .fn = TDH_SYS_LP_SHUTDOWN };
> > > > +
> > > > + seamcall_on_each_cpu(&sc);
> > > > +
> > > > tdx_module_status = TDX_MODULE_SHUTDOWN;
> > > > }
> > > >
> > > > @@ -221,6 +281,9 @@ bool platform_tdx_enabled(void)
> > > > * CPU hotplug is temporarily disabled internally to prevent any cpu
> > > > * from going offline.
> > > > *
> > > > + * Caller also needs to guarantee all CPUs are in VMX operation during
> > > > + * this function, otherwise Oops may be triggered.
> > >
> > > I would *MUCH* rather have this be a:
> > >
> > > if (!cpu_feature_enabled(X86_FEATURE_VMX))
> > > WARN_ONCE("VMX should be on blah blah\n");
> > >
> > > than just plain oops. Even a pr_err() that preceded the oops would be
> > > nicer than an oops that someone has to go decode and then grumble when
> > > their binutils is too old that it can't disassemble the TDCALL.
> >
> > I can add this to seamcall():
> >
> > /*
> > * SEAMCALL requires CPU being in VMX operation otherwise it causes
> > #UD.
> > * Sanity check and return early to avoid Oops. Note cpu_vmx_enabled()
> > * actually only checks whether VMX is enabled but doesn't check
> > whether
> > * CPU is in VMX operation (VMXON is done). There's no way to check
> > * whether VMXON has been done, but currently enabling VMX and doing
> > * VMXON are always done together.
> > */
> > if (!cpu_vmx_enabled()) {
> > WARN_ONCE("CPU is not in VMX operation before making
> > SEAMCALL");
> > return -EINVAL;
> > }
> >
> > The reason I didn't do is I'd like to make seamcall() simple, that it only
> > returns TDX_SEAMCALL_VMFAILINVALID or the actual SEAMCALL leaf error. With
> > above, this function also returns kernel error code, which isn't good.
>
> I think you're missing the point. You wasted two lines of code on a
> *COMMENT* that doesn't actually help anyone decode an oops. You could
> have, instead, spent two lines on actual code that would have been just
> as good or better than a comment *AND* help folks looking at an oops.
>
> It's almost always better to do something actionable in code than to
> comment it, unless it's in some crazy fast path.

Agreed. Thanks.

>
> > Alternatively, we can always add EXTABLE to TDX_MODULE_CALL macro to handle #UD
> > and #GP by returning dedicated error codes (please also see my reply to previous
> > patch for the code needed to handle), in which case we don't need such check
> > here.
> >
> > Always handling #UD in TDX_MODULE_CALL macro also has another advantage: there
> > will be no Oops for #UD regardless the issue that "there's no way to check
> > whether VMXON has been done" in the above comment.
> >
> > What's your opinion?
>
> I think you should explore using the EXTABLE. Let's see how it looks.

I tried to wrote the code before. I didn't test but it should look like to
something below. Any comments?

diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index 4b75c930fa1b..4a97ca8eb14c 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -8,6 +8,7 @@
#include <asm/ptrace.h>
#include <asm/shared/tdx.h>

+#ifdef CONFIG_INTEL_TDX_HOST
/*
* SW-defined error codes.
*
@@ -18,6 +19,21 @@
#define TDX_SW_ERROR (TDX_ERROR | GENMASK_ULL(47, 40))
#define TDX_SEAMCALL_VMFAILINVALID (TDX_SW_ERROR | _UL(0xFFFF0000))

+/*
+ * Special error codes to indicate SEAMCALL #GP and #UD.
+ *
+ * SEAMCALL causes #GP when SEAMRR is not properly enabled by BIOS, and
+ * causes #UD when CPU is not in VMX operation. Define two separate
+ * error codes to distinguish the two cases so caller can be aware of
+ * what caused the SEAMCALL to fail.
+ *
+ * Bits 61:48 are reserved bits which will never be set by the TDX
+ * module. Borrow 2 reserved bits to represent #GP and #UD.
+ */
+#define TDX_SEAMCALL_GP (TDX_ERROR | GENMASK_ULL(48, 48))
+#define TDX_SEAMCALL_UD (TDX_ERROR | GENMASK_ULL(49, 49))
+#endif
+
#ifndef __ASSEMBLY__

/*
diff --git a/arch/x86/virt/vmx/tdx/tdxcall.S b/arch/x86/virt/vmx/tdx/tdxcall.S
index 49a54356ae99..7431c47258d9 100644
--- a/arch/x86/virt/vmx/tdx/tdxcall.S
+++ b/arch/x86/virt/vmx/tdx/tdxcall.S
@@ -1,6 +1,7 @@
/* SPDX-License-Identifier: GPL-2.0 */
#include <asm/asm-offsets.h>
#include <asm/tdx.h>
+#include <asm/asm.h>

/*
* TDCALL and SEAMCALL are supported in Binutils >= 2.36.
@@ -45,6 +46,7 @@
/* Leave input param 2 in RDX */

.if \host
+1:
seamcall
/*
* SEAMCALL instruction is essentially a VMExit from VMX root
@@ -57,9 +59,25 @@
* This value will never be used as actual SEAMCALL error code as
* it is from the Reserved status code class.
*/
- jnc .Lno_vmfailinvalid
+ jnc .Lseamcall_out
mov $TDX_SEAMCALL_VMFAILINVALID, %rax
-.Lno_vmfailinvalid:
+ jmp .Lseamcall_out
+2:
+ /*
+ * SEAMCALL caused #GP or #UD. By reaching here %eax contains
+ * the trap number. Check the trap number and set up the return
+ * value to %rax.
+ */
+ cmp $X86_TRAP_GP, %eax
+ je .Lseamcall_gp
+ mov $TDX_SEAMCALL_UD, %rax
+ jmp .Lseamcall_out
+.Lseamcall_gp:
+ mov $TDX_SEAMCALL_GP, %rax
+ jmp .Lseamcall_out
+
+ _ASM_EXTABLE_FAULT(1b, 2b)
+.Lseamcall_out




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