Subject: Re: [RFC PATCH V3 05/11] HV: Add ghcb hvcall support for SNP VM

From: Joerg Roedel <[email-masked]>
Date: Wed, 9 Jun 2021 14:49:19 +0200

On Sun, May 30, 2021 at 11:06:22AM -0400, Tianyu Lan wrote:
> +u64 hv_ghcb_hypercall(u64 control, void *input, void *output, u32 input_size)
> +{
> +	union hv_ghcb *hv_ghcb;
> +	void **ghcb_base;
> +	unsigned long flags;
> +
> +	if (!ms_hyperv.ghcb_base)
> +		return -EFAULT;
> +
> +	local_irq_save(flags);
> +	ghcb_base = (void **)this_cpu_ptr(ms_hyperv.ghcb_base);
> +	hv_ghcb = (union hv_ghcb *)*ghcb_base;
> +	if (!hv_ghcb) {
> +		local_irq_restore(flags);
> +		return -EFAULT;
> +	}
> +
> +	memset(hv_ghcb, 0x00, HV_HYP_PAGE_SIZE);
> +	hv_ghcb->ghcb.protocol_version = 1;
> +	hv_ghcb->ghcb.ghcb_usage = 1;
> +
> +	hv_ghcb->hypercall.outputgpa = (u64)output;
> +	hv_ghcb->hypercall.hypercallinput.asuint64 = 0;
> +	hv_ghcb->hypercall.hypercallinput.callcode = control;
> +
> +	if (input_size)
> +		memcpy(hv_ghcb->hypercall.hypercalldata, input, input_size);
> +
> +	VMGEXIT();

Also not NMI-safe. When you re-use the existing GHCB setup code from
from SEV-ES code, you can also use sev_es_get/put_ghcb() which takes
care of re-using a GHCB already in use.

Regards,

	Joerg