linux-kernel - Re: [PATCH v3 2/5] x86/pvh: Make PVH entrypoint PIC for x86-64

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Message-ID: <1f62d675-0a1b-4ee6-ab39-314b1e443a4b@suse.com>
Date: Mon, 16 Sep 2024 08:27:22 +0200
From: Juergen Gross <jgross@...e.com>
To: Jason Andryuk <jason.andryuk@....com>,
 Boris Ostrovsky <boris.ostrovsky@...cle.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 Stefano Stabellini <sstabellini@...nel.org>,
 Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Brian Gerst <brgerst@...il.com>
Cc: xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/5] x86/pvh: Make PVH entrypoint PIC for x86-64

On 23.08.24 21:36, Jason Andryuk wrote:
> The PVH entrypoint is 32bit non-PIC code running the uncompressed
> vmlinux at its load address CONFIG_PHYSICAL_START - default 0x1000000
> (16MB).  The kernel is loaded at that physical address inside the VM by
> the VMM software (Xen/QEMU).
> 
> When running a Xen PVH Dom0, the host reserved addresses are mapped 1-1
> into the PVH container.  There exist system firmwares (Coreboot/EDK2)
> with reserved memory at 16MB.  This creates a conflict where the PVH
> kernel cannot be loaded at that address.
> 
> Modify the PVH entrypoint to be position-indepedent to allow flexibility
> in load address.  Only the 64bit entry path is converted.  A 32bit
> kernel is not PIC, so calling into other parts of the kernel, like
> xen_prepare_pvh() and mk_pgtable_32(), don't work properly when
> relocated.
> 
> This makes the code PIC, but the page tables need to be updated as well
> to handle running from the kernel high map.
> 
> The UNWIND_HINT_END_OF_STACK is to silence:
> vmlinux.o: warning: objtool: pvh_start_xen+0x7f: unreachable instruction
> after the lret into 64bit code.
> 
> Signed-off-by: Jason Andryuk <jason.andryuk@....com>

Reviewed-by: Juergen Gross <jgross@...e.com>

Juergen

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