Re: [PATCH v2 2/4] x86/vmware: Add a header file for hypercall definitions

From: Thomas HellstrÃm (VMware)
Date: Tue Aug 27 2019 - 15:19:17 EST

On 8/27/19 5:44 PM, Borislav Petkov wrote:
On Fri, Aug 23, 2019 at 10:13:14AM +0200, Thomas HellstrÃm (VMware) wrote:
+ * The high bandwidth out call. The low word of edx is presumed to have the
+ * HB and OUT bits set.
+ */
+ ALTERNATIVE_2("movw $" VMWARE_HYPERVISOR_PORT_HB ", %%dx; rep outsb", \
Hmm, that looks fishy:

This call in vmw_port_hb_out(), for example, gets converted to the asm
below (I've left in the asm touching only rDX).

# drivers/gpu/drm/vmwgfx/vmwgfx_msg.c:160: VMW_PORT_HB_OUT(
movzwl 0(%rbp), %edx # channel_20(D)->channel_id, channel_20(D)->channel_id


sall $16, %edx #, tmp172
orl $3, %edx #, tmp173

this is adding channel_id and flags:

VMWARE_HYPERVISOR_HB | (channel->channel_id << 16) |


movslq %edx, %rdx # tmp173, tmp174

Here it is sign-extending it.

# 160 "drivers/gpu/drm/vmwgfx/vmwgfx_msg.c" 1
push %rbp;mov %r8, %rbp;# ALT: oldinstr2 # bp
movw $0x5659, %dx; rep outsb

And now here you're overwriting the low word of %edx. And now it


and the low word doesn't contain the 3, i.e., (VMWARE_HYPERVISOR_HB |
VMWARE_HYPERVISOR_OUT) anymore. And that's before you do the hypercall
so I'm guessing that cannot be right.


It should be correct. The flags VMWARE_HYPERVISOR_HB and VMWARE_HYPERVISOR_OUT are only valid for the vmcall / vmmcall versions.

For the legacy version, the direction is toggled by the instruction (in vs out) and LB vs HB is toggled by the port number (0x5658 vs 0x5659)

So in essence the low word definition of %edx is different in the two versions. I've chosen to use the new vmcall/vmmcall definition in the driver code.