Re: [PATCH v15 05/23] x86/virt/tdx: Handle SEAMCALL no entropy error in common code

[Isaku Yamahata]

On Fri, Nov 10, 2023 at 12:55:42AM +1300,
Kai Huang <kai.huang@xxxxxxxxx> wrote:

> Some SEAMCALLs use the RDRAND hardware and can fail for the same reasons
> as RDRAND.  Use the kernel RDRAND retry logic for them.
> 
> There are three __seamcall*() variants.  Do the SEAMCALL retry in common
> code and add a wrapper for each of them.
> 
> Signed-off-by: Kai Huang <kai.huang@xxxxxxxxx>
> Reviewed-by: Kirill A. Shutemov <kirll.shutemov@xxxxxxxxxxxxxxx>
> Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> ---
> 
> v14 -> v15:
>  - Added Sathy's tag.
> 
> v13 -> v14:
>  - Use real function sc_retry() instead of using macros. (Dave)
>  - Added Kirill's tag.
> 
> v12 -> v13:
>  - New implementation due to TDCALL assembly series.
> 
> ---
>  arch/x86/include/asm/tdx.h | 26 ++++++++++++++++++++++++++
>  1 file changed, 26 insertions(+)
> 
> diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
> index ea9a0320b1f8..f1c0c15469f8 100644
> --- a/arch/x86/include/asm/tdx.h
> +++ b/arch/x86/include/asm/tdx.h
> @@ -24,6 +24,11 @@
>  #define TDX_SEAMCALL_GP			(TDX_SW_ERROR | X86_TRAP_GP)
>  #define TDX_SEAMCALL_UD			(TDX_SW_ERROR | X86_TRAP_UD)
>  
> +/*
> + * TDX module SEAMCALL leaf function error codes
> + */
> +#define TDX_RND_NO_ENTROPY	0x8000020300000000ULL
> +
>  #ifndef __ASSEMBLY__
>  
>  /*
> @@ -84,6 +89,27 @@ u64 __seamcall(u64 fn, struct tdx_module_args *args);
>  u64 __seamcall_ret(u64 fn, struct tdx_module_args *args);
>  u64 __seamcall_saved_ret(u64 fn, struct tdx_module_args *args);
>  
> +#include <asm/archrandom.h>
> +
> +typedef u64 (*sc_func_t)(u64 fn, struct tdx_module_args *args);
> +
> +static inline u64 sc_retry(sc_func_t func, u64 fn,
> +			   struct tdx_module_args *args)
> +{
> +	int retry = RDRAND_RETRY_LOOPS;
> +	u64 ret;
> +
> +	do {
> +		ret = func(fn, args);
> +	} while (ret == TDX_RND_NO_ENTROPY && --retry);

This loop assumes that args isn't touched when TDX_RND_NO_ENTRYPOY is returned.
It's not true.  TDH.SYS.INIT() and TDH.SYS.LP.INIT() clear RCX, RDX, etc on
error including TDX_RND_NO_ENTRY.  Because TDH.SYS.INIT() takes RCX as input,
this wrapper doesn't work.  TDH.SYS.LP.INIT() doesn't use RCX, RDX ... as
input. So it doesn't matter.

Other SEAMCALLs doesn't touch registers on the no entropy error.
TDH.EXPORTS.STATE.IMMUTABLE(), TDH.IMPORTS.STATE.IMMUTABLE(), TDH.MNG.ADDCX(),
and TDX.MNG.CREATE().  TDH.SYS.INIT() is an exception.
-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxxxxxxxx>