Re: [Xen-devel] [PATCH v5 02/10] x86emul: support MOVDIRI insn

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 24/03/2020 12:29, Jan Beulich wrote:
> Note that SDM revision 070 doesn't specify exception behavior for
> ModRM.mod == 0b11; assuming #UD here.

Didn't I confirm this behaviour for you last time around?

> @@ -10075,6 +10079,14 @@ x86_emulate(
>                              : "0" ((uint32_t)src.val), "rm" (_regs.edx) );
>          break;
>  
> +    case X86EMUL_OPC(0x0f38, 0xf9): /* movdiri mem,r */
> +        vcpu_must_have(movdiri);
> +        generate_exception_if(dst.type != OP_MEM, EXC_UD);
> +        /* Ignore the non-temporal behavior for now. */
> +        dst.val = src.val;
> +        sfence = true;

Looking again at the SDM, I'm not entirely sure this is good enough.

Even on top of WB/WP mappings, it needs to have WC properties, knock
aliasing lines out of the cache, and ending up as a bus transaction.

Also, I'm not convinced the current chunking algorithm for qemu which
repeatedly subdivides down to 1, is compatible with the misaligned
behaviour described, guaranteeing a split of two.

~Andrew