Re: [PATCH v3] hw/i386: place setup_data at fixed place in memory

From: Ard Biesheuvel <ardb@kernel.org>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>,
	qemu-devel@nongnu.org, "Xiaoyao Li" <xiaoyao.li@intel.com>,
	"Richard Henderson" <richard.henderson@linaro.org>,
	"Peter Maydell" <peter.maydell@linaro.org>,
	"Michael S . Tsirkin" <mst@redhat.com>,
	"Daniel P . Berrangé" <berrange@redhat.com>,
	"Gerd Hoffmann" <kraxel@redhat.com>,
	"Laszlo Ersek" <lersek@redhat.com>,
	linux-efi@vger.kernel.org
Subject: Re: [PATCH v3] hw/i386: place setup_data at fixed place in memory
Date: Fri, 5 Aug 2022 13:08:43 +0200	[thread overview]
Message-ID: <CAMj1kXFDs8HCCCcVAVwjLcATh6MYcUSAha5yvi0ftMw+Ddy_Xg@mail.gmail.com> (raw)
In-Reply-To: <40fdfb11-1e40-a36a-d3a4-fcbef546a78a@redhat.com>

On Fri, 5 Aug 2022 at 10:10, Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> On 8/5/22 01:04, Jason A. Donenfeld wrote:
> > +    /* Nothing else uses this part of the hardware mapped region */
> > +    setup_data_base = 0xfffff - 0x1000;
>
> Isn't this where the BIOS lives?  I don't think this works.
>
> Does it work to place setup_data at the end of the cmdline file instead
> of having it at the end of the kernel file?  This way the first item
> will be at 0x20000 + cmdline_size.
>

Does QEMU always allocate the command line statically like that?
AFAIK, OVMF never accesses that memory to read the command line, it
uses fw_cfg to copy it into a buffer it allocates itself. And I guess
that implies that this region could be clobbered by OVMF unless it is
told to preserve it.