Re: [PATCH v1 1/3] x86/traps: Move DR7_RESET_VALUE to <uapi/asm/debugreg.h>

[Sean Christopherson <seanjc@google.com>]

On Fri, Jun 13, 2025, Xin Li (Intel) wrote:
> Move DR7_RESET_VALUE to <uapi/asm/debugreg.h> to prepare to write DR7
> with DR7_RESET_VALUE at boot time.

Alternatively, what about dropping DR7_RESET_VALUE,  moving KVM's DR6 and DR7
#defines out of arch/x86/include/asm/kvm_host.h, and then using DR7_FIXED_1?

Arguably, that'd be an improvement for 2 of the 3 uses of DR7_RESET_VALUE in SEV
code:

	/* Early non-zero writes to DR7 are not supported */
	if (!data && (val & ~DR7_RESET_VALUE))
		return ES_UNSUPPORTED;

vs.

	/* Early non-zero writes to DR7 are not supported */
	if (!data && (val & ~DR7_FIXED_1))
		return ES_UNSUPPORTED;

And in vc_handle_dr7_read():

	if (data)
		*reg = data->dr7;
	else
		*reg = DR7_RESET_VALUE;

vs.

	if (data)
		*reg = data->dr7;
	else
		*reg = DR7_FIXED_1;

In both of those cases, it isn't the RESET value that's interesting, it's that
architecturally bit 10 is fixed to '1'.

I haven't looked at the kernel code, but I suspect DR6_ACTIVE_LOW, DR6_VOLATILE,
and/or DR6_FIXED_1 could also come in handy.