Subject: Re: [PATCH 05/13] HV: Add Write/Read MSR registers via ghcb page

From: Joerg Roedel <[email-masked]>
Date: Mon, 2 Aug 2021 14:28:36 +0200

On Wed, Jul 28, 2021 at 10:52:20AM -0400, Tianyu Lan wrote:
> +void hv_ghcb_msr_write(u64 msr, u64 value)
> +{
> +	union hv_ghcb *hv_ghcb;
> +	void **ghcb_base;
> +	unsigned long flags;
> +
> +	if (!ms_hyperv.ghcb_base)
> +		return;
> +
> +	WARN_ON(in_nmi());
> +
> +	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;
> +	}
> +
> +	memset(hv_ghcb, 0x00, HV_HYP_PAGE_SIZE);

Do you really need to zero out the whole 4k? The validation bitmap
should be enough, there are no secrets on the page anyway.
Same in hv_ghcb_msr_read().

> +enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb,
> +				   struct es_em_ctxt *ctxt,
> +				   u64 exit_code, u64 exit_info_1,
> +				   u64 exit_info_2)
>  {
>  	enum es_result ret;
>  
> @@ -109,7 +109,16 @@ static enum es_result sev_es_ghcb_hv_call(struct ghcb *ghcb,
>  	ghcb_set_sw_exit_info_1(ghcb, exit_info_1);
>  	ghcb_set_sw_exit_info_2(ghcb, exit_info_2);
>  
> -	sev_es_wr_ghcb_msr(__pa(ghcb));
> +	/*
> +	 * Hyper-V runs paravisor with SEV. Ghcb page is allocated by
> +	 * paravisor and not needs to be updated in the Linux guest.
> +	 * Otherwise, the ghcb page's PA reported by paravisor is above
> +	 * VTOM. Hyper-V use this function with NULL for ctxt point and
> +	 * skip setting ghcb page in such case.
> +	 */
> +	if (ctxt)
> +		sev_es_wr_ghcb_msr(__pa(ghcb));

No, do not make this function work with ctxt==NULL. Instead, factor out
a helper function which contains what Hyper-V needs and use that in
sev_es_ghcb_hv_call() and Hyper-V code.

> +union hv_ghcb {
> +	struct ghcb ghcb;
> +} __packed __aligned(PAGE_SIZE);

I am curious what this will end up being good for.