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Re: [PATCH v3 2/5] x86/pvh: Make PVH entrypoint PIC for x86-64
- To: Jason Andryuk <jason.andryuk@xxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, x86@xxxxxxxxxx, "H. Peter Anvin" <hpa@xxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>, Paolo Bonzini <pbonzini@xxxxxxxxxx>, Brian Gerst <brgerst@xxxxxxxxx>
- From: Juergen Gross <jgross@xxxxxxxx>
- Date: Mon, 16 Sep 2024 08:27:22 +0200
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx
- Delivery-date: Mon, 16 Sep 2024 06:27:30 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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@xxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Juergen
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