linux-kernel - Re: [PATCH v10 056/108] KVM: TDX: don't request KVM_REQ_APIC_PAGE

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Date:   Thu, 15 Dec 2022 16:11:57 -0800
From:   Isaku Yamahata <isaku.yamahata@...il.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Shahar, Sagi" <sagis@...gle.com>,
        "Aktas, Erdem" <erdemaktas@...gle.com>,
        "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "dmatlack@...gle.com" <dmatlack@...gle.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>
Subject: Re: [PATCH v10 056/108] KVM: TDX: don't request
 KVM_REQ_APIC_PAGE_RELOAD

On Mon, Nov 21, 2022 at 11:55:58PM +0000,
"Huang, Kai" <kai.huang@...el.com> wrote:

> On Sat, 2022-10-29 at 23:22 -0700, isaku.yamahata@...el.com wrote:
> > From: Isaku Yamahata <isaku.yamahata@...el.com>
> > 
> > TDX doesn't need APIC page depending on vapic and its callback is
> > WARN_ON_ONCE(is_tdx).  To avoid unnecessary overhead and WARN_ON_ONCE(),
> > skip requesting KVM_REQ_APIC_PAGE_RELOAD when TD.
> > 
> >   WARNING: arch/x86/kvm/vmx/main.c:696 vt_set_apic_access_page_addr+0x3c/0x50 [kvm_intel]
> >   RIP: 0010:vt_set_apic_access_page_addr+0x3c/0x50 [kvm_intel]
> >   Call Trace:
> >    vcpu_enter_guest+0x145d/0x24d0 [kvm]
> >    kvm_arch_vcpu_ioctl_run+0x25d/0xcc0 [kvm]
> >    kvm_vcpu_ioctl+0x414/0xa30 [kvm]
> >    __x64_sys_ioctl+0xc0/0x100
> >    do_syscall_64+0x39/0xc0
> >    entry_SYSCALL_64_after_hwframe+0x44/0xae
> > 
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> > ---
> >  arch/x86/kvm/x86.c | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 3868605462ed..5dadd0f9a10e 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -10487,7 +10487,9 @@ void kvm_arch_mmu_notifier_invalidate_range(struct kvm *kvm,
> >  	 * Update it when it becomes invalid.
> >  	 */
> >  	apic_address = gfn_to_hva(kvm, APIC_DEFAULT_PHYS_BASE >> PAGE_SHIFT);
> > -	if (start <= apic_address && apic_address < end)
> > +	/* TDX doesn't need APIC page. */
> > +	if (kvm->arch.vm_type != KVM_X86_TDX_VM &&
> > +	    start <= apic_address && apic_address < end)
> >  		kvm_make_all_cpus_request(kvm, KVM_REQ_APIC_PAGE_RELOAD);
> >  }
> >  
> 
> In patch "[PATCH v10 105/108] KVM: TDX: Add methods to ignore accesses to CPU
> state", you have:
> 
> +static void vt_set_apic_access_page_addr(struct kvm_vcpu *vcpu)
> +{
> +	if (WARN_ON_ONCE(is_td_vcpu(vcpu)))
> +		return;
> +
> +	vmx_set_apic_access_page_addr(vcpu);
> +}
> 
> If you drop the WARN_ON_ONCE() above, you can just drop this patch.
> 
> For this particular case, I don't find it is quite necessary to change the
> common x86 code as done in this patch.  In fact, SVM doesn't have a
> set_apic_access_page_addr() callback which is consistent with just return if VM
> is TD in vt_set_apic_access_page_addr().
> 

Oh, yes. I will drop this patch with removing WARN_ON_ONCE().


> Also, I don't particularly like the idea of having a lot of "is_td(kvm)" in the
> common x86 code as if similar technology happens in the future, you will need to
> have another "is_td_similar_vm(kvm)" thing.

Currently KVM_CAP_VM_TYPES has such check in x86 kvm common code.


> If modifying common x86 code is necessary, then it would make more sense to
> introduce some common flag, and make TD guest set that flag.
> 
> Btw, this patch just comes out of blue from the  middle of a bunch of MMU
> patches.  Shouldn't it be moved to "patches which handles interrupt related
> staff"?
> 
> Btw2, by saying above, does it make sense to split patch "[PATCH v10 105/108]
> KVM: TDX: Add methods to ignore accesses to CPU state" based on category such as
> MMU/interrupt, etc?  Particularly, in that patch, some callbacks have WARN() or
> KVM_BUG_ON() against TD guest, but some don't.  The logic behind those decisions
> highly depend on previous patches.  To me, it makes more sense to just move
> logic related things together.

Ok, I'll split it up to cpu states/KVM MMU/interrupt parts.
-- 
Isaku Yamahata <isaku.yamahata@...il.com>