linux-kernel - Re: [PATCH v9 10/19] KVM: selftests: Set up TDX boot code region

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Message-ID: <aK3i910ghgsaIkir@google.com>
Date: Tue, 26 Aug 2025 09:38:15 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Sagi Shahar <sagis@...gle.com>, linux-kselftest@...r.kernel.org, 
	Paolo Bonzini <pbonzini@...hat.com>, Shuah Khan <shuah@...nel.org>, 
	Ackerley Tng <ackerleytng@...gle.com>, Ryan Afranji <afranji@...gle.com>, 
	Andrew Jones <ajones@...tanamicro.com>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	Erdem Aktas <erdemaktas@...gle.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	Roger Wang <runanwang@...gle.com>, Binbin Wu <binbin.wu@...ux.intel.com>, 
	Oliver Upton <oliver.upton@...ux.dev>, "Pratik R. Sampat" <pratikrajesh.sampat@....com>, 
	Reinette Chatre <reinette.chatre@...el.com>, Ira Weiny <ira.weiny@...el.com>, 
	Chao Gao <chao.gao@...el.com>, Chenyi Qiang <chenyi.qiang@...el.com>, 
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v9 10/19] KVM: selftests: Set up TDX boot code region

On Mon, Aug 25, 2025, Yan Zhao wrote:
> > +	/*
> > +	 * Handcode "JMP rel8" at the RESET vector to jump back to the TD boot
> > +	 * code, as there are only 16 bytes at the RESET vector before RIP will
> > +	 * wrap back to zero.  Insert a trailing int3 so that the vCPU crashes
> > +	 * in case the JMP somehow falls through.  Note!  The target address is
> > +	 * relative to the end of the instruction!
> > +	 */
> > +	TEST_ASSERT(TD_BOOT_CODE_SIZE < 256,
> Looks TD_BOOT_CODE_SIZE needs to be <= 126, as the jump range is limited to -128
> to +127 for JMP rel8.

Gah, I managed to forget that relative targets obviously need to be signed values,
and I also forgot to account for the size of the JMP in the assert.  Go me.

Maybe express this as:

	TEST_ASSERT(TD_BOOT_CODE_SIZE + 2 < 128,
		    "TD boot code not addressable by 'JMP rel8'");
	
> > +		    "TD boot code not addressable by 'JMP rel8'");
> > +	hva[0] = 0xeb;
> > +	hva[1] = 256 - 2 - TD_BOOT_CODE_SIZE;

I think I lucked into getting this right though?

> > +	hva[2] = 0xcc;
> > +}
> > -- 
> > 2.51.0.rc1.193.gad69d77794-goog
> > 
> >