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Tue, 12 May 2026 07:59:36 -0700 (PDT) Message-ID: <383355d1-7032-4445-8a06-cb4411ea797e@suse.com> Date: Tue, 12 May 2026 16:59:35 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: [PATCH for-4.22 3/5] x86/vRTC: support century field From: Jan Beulich To: "xen-devel@lists.xenproject.org" Cc: Andrew Cooper , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Teddy Astie , Oleksii Kurochko References: Content-Language: en-US Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-purgate-ID: tlsNG-33051d/1778597977-4086B938-1FA6D66E/0/0 X-purgate-type: clean X-purgate-size: 8908 Both ROMBIOS and SeaBIOS (with CONFIG_QEMU=y, as we build it) blindly assume availability of this field (at its conventional index 0x32); OVMF at least has code to inspect FADT. Hence we ought to have supported it virtually forever. As the index is beyond RTC_CMOS_SIZE, leverage the padding field in struct hvm_hw_rtc to hold its value. Update the field only when involved values are valid BCD century specifiers. Otherwise (for VMs migrated in from an older hypervisor) leave handling to the DM. This makes the Linux rtc-cmos driver report y3k compatibility. While extending xen-hvmctx.c:dump_rtc() also add RTC offset there. Fixes: 4ca161214355 ("[HVM] Move RTC emulation into the hypervisor") Signed-off-by: Jan Beulich --- Am I overly paranoid with the checking of the field, considering that Xen 3.x post-dates year 2000 and hence all firmware nowadays usable guests have ever run with should have been aware of the field? Or am I, quite the opposite, still not strict enough? I can't help the impression that this introduces a latency issue for the 2nd of gmtime()'s while() loops: We now allow years up into the 99th century, i.e. over 8000 years away from 1970. 8000 years are very roughly 2^^38 seconds, making for (again very roughly) 5 million iterations there. Did I get my math wrong, or do we need a prereq change to (vastly) reduce the number of iterations of that loop (e.g. along the lines of the other one, first going in 400 year steps)? Isn't day-of-week handling flawed? If the field is brought out of sync with the other values, shouldn't it stay respectively out-of-sync? And isn't it excessive overhead to go through rtc_set_time() when the field is updated while SET is clear? Perhaps we ought to also support alarm day/month features? --- a/tools/libacpi/static_tables.c +++ b/tools/libacpi/static_tables.c @@ -33,6 +33,8 @@ struct acpi_20_facs Facs = { #define ACPI_PM_TMR_BLK_BIT_WIDTH 0x20 #define ACPI_PM_TMR_BLK_BIT_OFFSET 0x00 +#define CMOS_CENTURY 0x32 /* Conventional index used also without ACPI */ + struct acpi_fadt Fadt = { .header = { .signature = ACPI_FADT_SIGNATURE, @@ -88,7 +90,9 @@ struct acpi_fadt Fadt = { .register_bit_width = ACPI_PM_TMR_BLK_BIT_WIDTH, .register_bit_offset = ACPI_PM_TMR_BLK_BIT_OFFSET, .address = ACPI_PM_TMR_BLK_ADDRESS_V1, - } + }, + + .century = CMOS_CENTURY, }; struct acpi_20_rsdt Rsdt = { --- a/tools/misc/xen-hvmctx.c +++ b/tools/misc/xen-hvmctx.c @@ -311,7 +311,7 @@ static void dump_rtc(void) printf(" 0x%2.2x 0x%2.2x 0x%2.2x 0x%2.2x 0x%2.2x 0x%2.2x, index 0x%2.2x\n", r.cmos_data[8], r.cmos_data[9], r.cmos_data[10], r.cmos_data[11], r.cmos_data[12], r.cmos_data[13], r.cmos_index); - + printf(" century 0x%02x offset %"PRId64"\n", r.century, r.rtc_offset); } static void dump_hpet(void) --- a/xen/arch/x86/hvm/rtc.c +++ b/xen/arch/x86/hvm/rtc.c @@ -47,6 +47,12 @@ #define epoch_year 1900 #define get_year(x) ((x) + epoch_year) +static inline bool is_century(unsigned int x) +{ + /* Constant below should match epoch_year above, just as BCD value. */ + return x >= 0x19 && (x & 0xf) < 10 && (x >> 4) < 10; +} + enum rtc_mode { rtc_mode_no_ack, rtc_mode_strict @@ -482,16 +488,32 @@ static int rtc_ioport_write(void *opaque data &= 0x7f; s->hw.cmos_index = data; spin_unlock(&s->lock); + /* RTC_CENTURY always forwarded to DM. */ return (data < RTC_CMOS_SIZE); } - if ( s->hw.cmos_index >= RTC_CMOS_SIZE ) + switch ( s->hw.cmos_index ) { + case 0 ... RTC_CMOS_SIZE - 1: + orig = s->hw.cmos_data[s->hw.cmos_index]; + break; + + case RTC_CENTURY: + orig = s->hw.century; + if ( !is_century(orig) || !is_century(data) ) + { + /* Prevent further use of the field. */ + s->hw.century = 0; + spin_unlock(&s->lock); + return 0; + } + break; + + default: spin_unlock(&s->lock); return 0; } - orig = s->hw.cmos_data[s->hw.cmos_index]; switch ( s->hw.cmos_index ) { case RTC_SECONDS_ALARM: @@ -507,6 +529,7 @@ static int rtc_ioport_write(void *opaque case RTC_DAY_OF_MONTH: case RTC_MONTH: case RTC_YEAR: + case RTC_CENTURY: /* if in set mode, just write the register */ if ( (s->hw.cmos_data[RTC_REG_B] & RTC_SET) ) s->hw.cmos_data[s->hw.cmos_index] = data; @@ -515,7 +538,10 @@ static int rtc_ioport_write(void *opaque /* Fetch the current time and update just this field. */ s->current_tm = gmtime(get_localtime(d)); rtc_copy_date(s); - s->hw.cmos_data[s->hw.cmos_index] = data; + if ( s->hw.cmos_index != RTC_CENTURY ) + s->hw.cmos_data[s->hw.cmos_index] = data; + else + s->hw.century = data; rtc_set_time(s); } alarm_timer_update(s); @@ -591,7 +617,16 @@ static void rtc_set_time(RTCState *s) tm->tm_wday = from_bcd(s, s->hw.cmos_data[RTC_DAY_OF_WEEK]); tm->tm_mday = from_bcd(s, s->hw.cmos_data[RTC_DAY_OF_MONTH]); tm->tm_mon = from_bcd(s, s->hw.cmos_data[RTC_MONTH]) - 1; - tm->tm_year = from_bcd(s, s->hw.cmos_data[RTC_YEAR]) + 100; + tm->tm_year = from_bcd(s, s->hw.cmos_data[RTC_YEAR]); + if ( is_century(s->hw.century) ) + { + unsigned int century = s->hw.century; + + BCD_TO_BIN(century); + tm->tm_year += century * 100 - epoch_year; + } + else + tm->tm_year += 100; after = mktime(get_year(tm->tm_year), tm->tm_mon + 1, tm->tm_mday, tm->tm_hour, tm->tm_min, tm->tm_sec); @@ -629,6 +664,12 @@ static void rtc_copy_date(RTCState *s) s->hw.cmos_data[RTC_DAY_OF_MONTH] = to_bcd(s, tm->tm_mday); s->hw.cmos_data[RTC_MONTH] = to_bcd(s, tm->tm_mon + 1); s->hw.cmos_data[RTC_YEAR] = to_bcd(s, tm->tm_year % 100); + + if ( is_century(s->hw.century) ) + { + s->hw.century = get_year(tm->tm_year) / 100; + BIN_TO_BCD(s->hw.century); + } } static int update_in_progress(RTCState *s) @@ -656,6 +697,13 @@ static uint32_t rtc_ioport_read(RTCState switch ( s->hw.cmos_index ) { + case RTC_CENTURY: + if ( !is_century(s->hw.century) ) + { + ret = UINT32_MAX; + break; + } + fallthrough; case RTC_SECONDS: case RTC_MINUTES: case RTC_HOURS: @@ -669,7 +717,10 @@ static uint32_t rtc_ioport_read(RTCState s->current_tm = gmtime(get_localtime(d)); rtc_copy_date(s); } - ret = s->hw.cmos_data[s->hw.cmos_index]; + if ( s->hw.cmos_index != RTC_CENTURY ) + ret = s->hw.cmos_data[s->hw.cmos_index]; + else + ret = s->hw.century; break; case RTC_REG_A: ret = s->hw.cmos_data[s->hw.cmos_index]; @@ -718,10 +769,12 @@ static int cf_check handle_rtc_io( *val = 0xff; return X86EMUL_OKAY; } - else if ( vrtc->hw.cmos_index < RTC_CMOS_SIZE ) + else if ( vrtc->hw.cmos_index < RTC_CMOS_SIZE || + vrtc->hw.cmos_index == RTC_CENTURY ) { *val = rtc_ioport_read(vrtc); - return X86EMUL_OKAY; + if ( *val != UINT32_MAX ) + return X86EMUL_OKAY; } return X86EMUL_UNHANDLEABLE; @@ -873,6 +926,8 @@ void rtc_init(struct domain *d) s->hw.cmos_data[RTC_REG_C] = 0; s->hw.cmos_data[RTC_REG_D] = RTC_VRT; + s->hw.century = 0x20; /* Arbitrary initial value satisfying is_century() */ + s->current_tm = gmtime(get_localtime(d)); s->start_time = NOW(); --- a/xen/arch/x86/include/asm/mc146818rtc.h +++ b/xen/arch/x86/include/asm/mc146818rtc.h @@ -37,6 +37,9 @@ bool is_cmos_port(unsigned int port, uns #define RTC_REG_C 12 #define RTC_REG_D 13 +/* Conventional index used without (and typically also with) ACPI. */ +#define RTC_CENTURY 0x32 + /********************************************************************** * register details **********************************************************************/ --- a/xen/include/public/arch-x86/hvm/save.h +++ b/xen/include/public/arch-x86/hvm/save.h @@ -485,7 +485,7 @@ struct hvm_hw_rtc { uint8_t cmos_data[RTC_CMOS_SIZE]; /* Index register for 2-part operations */ uint8_t cmos_index; - uint8_t pad0; + uint8_t century; /* RTC offset from host time */ int64_t rtc_offset; };