Re: [PATCH] x86/tdx: Fix crash on kexec with CONFIG_EISA

[Kirill A. Shutemov]

On Sat, Aug 24, 2024 at 11:29:39PM +0100, Maciej W. Rozycki wrote:
> On Thu, 22 Aug 2024, Kirill A. Shutemov wrote:
> 
> > This issue causes real problems:
> > 
> > 1. If the kernel is compiled with EISA support, it will attempt to probe
> >    EISA by reading 4 bytes from the 0x0FFFD9 address (see eisa_bus_probe()).
> >    The kernel treats this read as MMIO and accesses this memory via
> >    shared mapping as we do for MMIO.
> > 
> >    KVM converts memory to shared upon such access.
> > 
> > 2. The same memory range (0xF0000-0x100000) is scanned to look for the MP
> >    table (see mpparse_find_mptable()). However, this is not MMIO and it
> >    is accessed via private mapping.
> > 
> >    This will cause a crash if the memory is not private.
> > 
> > During normal boot, the kernel scans for SMP information before probing
> > for EISA, and it boots fine. However, the memory becomes shared and causes
> > issues on kexec when the second kernel attempts to scan for SMP information.
> 
>  ISTM that `eisa_bus_probe' has to be updated to `memremap' analogously to 
> `mpparse_find_mptable', complementing changes such as commit f7750a795687 
> ("x86, mpparse, x86/acpi, x86/PCI, x86/dmi, SFI: Use memremap() for RAM 
> mappings") or commit 5997efb96756 ("x86/boot: Use memremap() to map the 
> MPF and MPC data").  Both just access BIOS memory.
> 
>  Can you please try and verify if my proposed change at: 
> <https://lore.kernel.org/r/alpine.DEB.2.21.2408242025210.30766@xxxxxxxxxxxxxxxxx> 
> has fixed the problem for you?

I like the direction your patch took. I hate sprinkling
X86_FEATURE_TDX_GUEST checks over the kernel.

Unfortunately, it is not enough to fix the issue. memremap() in this case
will still boil down to ioremap() that would set shared bit:

memremap()
  arch_memremap_wb()
    ioremap_cache()
      __ioremap_caller(.encrytped = false)

I think arch_memremap_wb() should be mapped ioremap_encrypted() in x86
case. See the patch below.

It seems to be working fine on TDX, but I am not sure about SEV.

Tom, any comments?

diff --git a/arch/x86/include/asm/io.h b/arch/x86/include/asm/io.h
index 1d60427379c9..ef79cbef1ef8 100644
--- a/arch/x86/include/asm/io.h
+++ b/arch/x86/include/asm/io.h
@@ -180,6 +180,8 @@ extern void __iomem *ioremap_prot(resource_size_t offset, unsigned long size, un
 extern void __iomem *ioremap_encrypted(resource_size_t phys_addr, unsigned long size);
 #define ioremap_encrypted ioremap_encrypted
 
+#define arch_memremap_wb ioremap_encrypted
+
 /**
  * ioremap     -   map bus memory into CPU space
  * @offset:    bus address of the memory
-- 
  Kiryl Shutsemau / Kirill A. Shutemov