Re: [PATCH 0/3] x86/pvh: fix unbootable VMs again (PVH + KASAN)

[Mauricio Faria de Oliveira <mfo@igalia.com>]

On 2026-04-22 17:07, Mauricio Faria de Oliveira wrote:
> The issue of unbootable VMs with CONFIG_PVH due to CONFIG_KASAN is back.
> 
> Booting directly from vmlinux (instead of bzImage) now fails with gcc-14/15
> (but works with gcc-12/13) if CONFIG_KASAN_GENERIC is set, on Ubuntu 25.10.
> 
> The PVH code is required/supposed not to hit the KASAN memory access check
> in the kernel entry point as KASAN has not yet been setup, or an exception
> is hit and the boot fails.
> 
> This was previously described and addressed with __builtin_mem{cmp,set}():
> - commit 661362e3dcab ("xen, pvh: fix unbootable VMs (PVH + KASAN - AMD_MEM_ENCRYPT)")
> - commit 416a33c9afce ("x86/cpu: fix unbootable VMs by inlining memcmp() in hypervisor_cpuid_base()")
> - commit fbe5a6dfe492 ("xen, pvh: fix unbootable VMs by inlining memset() in xen_prepare_pvh()")
> 
> However, even with __builtin the compiler may decide to use the out of line
> function instead of the inline implementation. So, that does not really fix
> the issue unconditionally, as it's being seen (details below).
> 
> In order to address this, it's required to switch to inline implementations
> that do not depend on the compiler.
> 
> There's such a memset in <asm/string.h> and memcmp in 'boot/string.c', now
> exposed in <asm/string.h> too. Use them instead of builtins in PVH entry.
[...]

v2:
https://lore.kernel.org/r/20260427-pvh-kasan-inline-v2-0-2c57b8dcff6a@igalia.com

-- 
Mauricio