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

[Tianyu Lan]

On 8/2/2021 8:28 PM, Joerg Roedel wrote:
> 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().

OK. Thanks for suggestion. I will have a try.

> > +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.

OK. Will update.

> > +union hv_ghcb {
> > +	struct ghcb ghcb;
> > +} __packed __aligned(PAGE_SIZE);
>
> I am curious what this will end up being good for.

Hyper-V introduces a specific hypercall request in GHCB page and use 
same union in the Linux Hyper-V code to read/write MSR and call the new 
hypercall request.