Currently attempt to do MMIO from userspace in TDX guest leads to
warning about unexpect #VE and SIGSEGV being delivered to the process.

Enlightened userspace might choose to deal with MMIO on their own if
kernel doesn't emulate it.

Handle EPT_VIOLATION exit reason for userspace and deliver SIGBUS
instead of SIGSEV. SIGBUS is more appropriate for MMIO situation.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 arch/x86/coco/tdx/tdx.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index c1cb90369915..d2aa93cebf5a 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -7,6 +7,7 @@
 #include <linux/cpufeature.h>
 #include <linux/export.h>
 #include <linux/io.h>
+#include <linux/sched/signal.h>
 #include <asm/coco.h>
 #include <asm/tdx.h>
 #include <asm/vmx.h>
@@ -630,6 +631,11 @@ void tdx_get_ve_info(struct ve_info *ve)
 	ve->instr_info  = upper_32_bits(args.r10);
 }
 
+static inline bool is_private_gpa(u64 gpa)
+{
+	return gpa == cc_mkenc(gpa);
+}
+
 /*
  * Handle the user initiated #VE.
  *
@@ -641,17 +647,20 @@ static int virt_exception_user(struct pt_regs *regs, struct ve_info *ve)
 	switch (ve->exit_reason) {
 	case EXIT_REASON_CPUID:
 		return handle_cpuid(regs, ve);
+	case EXIT_REASON_EPT_VIOLATION:
+		if (is_private_gpa(ve->gpa))
+			panic("Unexpected EPT-violation on private memory.");
+
+		force_sig_fault(SIGBUS, BUS_ADRERR, (void __user *)ve->gla);
+
+		/* Return 0 to avoid incrementing RIP */
+		return 0;
 	default:
 		pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
 		return -EIO;
 	}
 }
 
-static inline bool is_private_gpa(u64 gpa)
-{
-	return gpa == cc_mkenc(gpa);
-}
-
 /*
  * Handle the kernel #VE.
  *
-- 
2.43.0

On 5/21/24 12:35 AM, Kirill A. Shutemov wrote:
> Currently attempt to do MMIO from userspace in TDX guest leads to
> warning about unexpect #VE and SIGSEGV being delivered to the process.
>
> Enlightened userspace might choose to deal with MMIO on their own if
> kernel doesn't emulate it.

Any specific use cases ? Like who is using it?

> Handle EPT_VIOLATION exit reason for userspace and deliver SIGBUS
> instead of SIGSEV. SIGBUS is more appropriate for MMIO situation.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---

Code looks good to me.

Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>

>  arch/x86/coco/tdx/tdx.c | 19 ++++++++++++++-----
>  1 file changed, 14 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
> index c1cb90369915..d2aa93cebf5a 100644
> --- a/arch/x86/coco/tdx/tdx.c
> +++ b/arch/x86/coco/tdx/tdx.c
> @@ -7,6 +7,7 @@
>  #include <linux/cpufeature.h>
>  #include <linux/export.h>
>  #include <linux/io.h>
> +#include <linux/sched/signal.h>
>  #include <asm/coco.h>
>  #include <asm/tdx.h>
>  #include <asm/vmx.h>
> @@ -630,6 +631,11 @@ void tdx_get_ve_info(struct ve_info *ve)
>  	ve->instr_info  = upper_32_bits(args.r10);
>  }
>  
> +static inline bool is_private_gpa(u64 gpa)
> +{
> +	return gpa == cc_mkenc(gpa);
> +}
> +
>  /*
>   * Handle the user initiated #VE.
>   *
> @@ -641,17 +647,20 @@ static int virt_exception_user(struct pt_regs *regs, struct ve_info *ve)
>  	switch (ve->exit_reason) {
>  	case EXIT_REASON_CPUID:
>  		return handle_cpuid(regs, ve);
> +	case EXIT_REASON_EPT_VIOLATION:
> +		if (is_private_gpa(ve->gpa))
> +			panic("Unexpected EPT-violation on private memory.");
> +
> +		force_sig_fault(SIGBUS, BUS_ADRERR, (void __user *)ve->gla);
> +
> +		/* Return 0 to avoid incrementing RIP */
> +		return 0;
>  	default:
>  		pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
>  		return -EIO;
>  	}
>  }
>  
> -static inline bool is_private_gpa(u64 gpa)
> -{
> -	return gpa == cc_mkenc(gpa);
> -}
> -
>  /*
>   * Handle the kernel #VE.
>   *

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer

On Tue, May 21, 2024 at 06:35:49AM -0700, Kuppuswamy Sathyanarayanan wrote:
> 
> On 5/21/24 12:35 AM, Kirill A. Shutemov wrote:
> > Currently attempt to do MMIO from userspace in TDX guest leads to
> > warning about unexpect #VE and SIGSEGV being delivered to the process.
> >
> > Enlightened userspace might choose to deal with MMIO on their own if
> > kernel doesn't emulate it.
> 
> Any specific use cases ? Like who is using it?

Microsoft folks wanted it. Chris, Dexuan, John, any comments?

But it is generally right thing to do. SIGBUS is right signal to deliver.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov