linux-kernel - RE: [PATCH v2 5/6] x86/hyperv: Support hypercalls for TDX guests

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Message-ID: <SA1PR21MB133549756FCE0834498000CCBFFE9@SA1PR21MB1335.namprd21.prod.outlook.com>
Date:   Mon, 9 Jan 2023 07:27:54 +0000
From:   Dexuan Cui <decui@...rosoft.com>
To:     Zhi Wang <zhi.wang.linux@...il.com>
CC:     "ak@...ux.intel.com" <ak@...ux.intel.com>,
        "arnd@...db.de" <arnd@...db.de>, "bp@...en8.de" <bp@...en8.de>,
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        KY Srinivasan <kys@...rosoft.com>,
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Subject: RE: [PATCH v2 5/6] x86/hyperv: Support hypercalls for TDX guests

> From: Zhi Wang <zhi.wang.linux@...il.com>
> Sent: Friday, January 6, 2023 3:24 AM
> > @@ -83,6 +94,9 @@ static inline u64 hv_do_fast_hypercall8(u16 code, u64
> > input1) u64 hv_status, control = (u64)code | HV_HYPERCALL_FAST_BIT;
> >
> >  #ifdef CONFIG_X86_64
> > +	if (hv_isolation_type_tdx())
> > +		return hv_tdx_hypercall(control, input1, 0);
> > +
> >  	{
> >  		__asm__ __volatile__(CALL_NOSPEC
> >  				     : "=a" (hv_status),
> > ASM_CALL_CONSTRAINT, @@ -114,6 +128,9 @@ static inline u64
> > hv_do_fast_hypercall16(u16 code, u64 input1, u64 input2) u64 hv_status,
> > control = (u64)code | HV_HYPERCALL_FAST_BIT;
> >  #ifdef CONFIG_X86_64
> > +	if (hv_isolation_type_tdx())
> > +		return hv_tdx_hypercall(control, input1, input2);
> > +
> In some paths, for example vmbus_set_event(), choosing the SNP-based or

In a SNP guest with pavavisor on Hyper-V, hv_ghcb_hypercall() is called in
only two places: vmbus_set_event() and hv_post_message(), where the
hypercalls, which are done via GHCB, need to be handled by the Hyper-V
hypervisor directly; in other places, the hypercalls, which are done via the
hypercall page, need to be handled by the pavavisor. That's to say, there
are 2 different kinds of hypercalls for a SNP guest with pavavisor on Hyper-V,
and hence we have to use 2 styles.

> generic hypercall happens in the caller, while now TDX-based hypercall is
> embraced in the generic hypercall path, e.g. hv_do_fast_hypercall8(). Which
> style will be chosen in the future? Seems the coding structure needs to be
> aligned.

For a TDX guest without pavavisor on Hyper-V, there is only one style of
hypercalls, so I make the change in hv_do_hypercall() and
hv_do_fast_hypercall*() directly. 

I don't think we can make any clean-up changes right now. When we
support the TDX guest with pavavisor on Hyper-V, we'll figure out if we
can make any improvement.

> void vmbus_set_event(struct vmbus_channel *channel)
> {
>         u32 child_relid = channel->offermsg.child_relid;
> 
>         if (!channel->is_dedicated_interrupt)
>                 vmbus_send_interrupt(child_relid);
> 
>         ++channel->sig_events;
> 
>         if (hv_isolation_type_snp())
>                 hv_ghcb_hypercall(HVCALL_SIGNAL_EVENT,
> &channel->sig_event,
>                                 NULL, sizeof(channel->sig_event));
>         else
>                 hv_do_fast_hypercall8(HVCALL_SIGNAL_EVENT,
>         channel->sig_event);
> }
> 
> 
> >  	{
> >  		__asm__ __volatile__("mov %4, %%r8\n"
> >  				     CALL_NOSPEC